Virus Transmission and Epidemiology (2024)

Abstract

For transmission of a virus to occur, a virus must enter a host through a portal of entry, replicate or disseminate within the host, and be transmitted to a new host through a portal of exit. Unless delivered directly into bodily tissues through a bite or needle, most viruses interact with the epithelium at the site of entry. Localized infections replicate at the initial site of infection, while systemic infections spread to additional areas of the body. Viruses are shed into the environment most often through the same route they entered the body. The stability of virions within the environment is dependent upon virion and environmental factors. Epidemiology is the study of how diseases are transmitted through a population. Epidemiologists perform descriptive or analytic studies to characterize the chain of viral infection throughout a population and design control measures to interrupt it.

Keywords: Case definition, Congenital infection, Epidemic, Epidemiology, Epithelium, Fecal–oral route, Gastrointestinal tract, Infection, Pandemic, Respiratory tract, Sexually transmitted diseases, Shedding, Skin, Transmission, Vertical transmission, Viral entry, Viremia

The previous chapter described how viruses enter, replicate within, and exit host cells. This chapter takes a broader look at how viruses enter a host, spread throughout the body, and exit a host to infect other individuals within a population.

Viral pathogenesis is how viruses cause disease within a host. Several factors must be overcome, however, for a virus to initiate a successful infection. First, sufficient numbers of virions must enter the host. A single virion is theoretically enough to initiate infection, but there are many other factors that make it unlikely that a single virion will be successful in establishing an infection. The host cells must be accessible to the virus, and those cells must be susceptible to infection, meaning that the cells express the receptors to which the virus can bind. This affinity for susceptible tissues is known as tropism. The cells must also be permissive to infection, meaning that they contain the proteins and molecules within the cell that are necessary for replication to occur. There are also mechanical, chemical, and microbiological barriers to infection at every site within the body, and the host’s immune system is quickly activated to eradicate the virus. The viruses that exist today have evolutionarily been selected for their traits that allow them to circumvent host factors and initiate infection, although the most successful viruses are not the most virulent: an extremely pathogenic virus will kill its host, thereby eliminating its reservoir and interrupting the chain of infection to another susceptible host.

5.1. Portals of Virus Entry

To establish infection, a virus must come in contact with host cells that are susceptible and permissive to infection. There are several different portals of entry that are used by different viruses (Fig. 5.1and Table 5.1). Most viruses interact with the cells of the host epithelium, the layers of cells that line the outside surface and inner cavities of the body. The epithelium acts as the main barrier between the outside world and the internal environment of the body. Mucosal epithelium, so named because the epithelium is covered in a protective layer of mucus, lines all the internal surfaces of the body, including the respiratory tract, gastrointestinal tract, and genital tract. The epithelium can be bypassed, however, when viruses are delivered to internal sites through penetration of the skin, as happens with an insect or animal bite, or through transplantation of a virally infected organ. Several viruses are also able to cross the placenta to a fetus or be transmitted to a child during or after birth.

5.2. Dissemination Within a Host

As we have seen, viruses can gain entry into a host through a variety of ways. Viruses that infect and replicate only within cells at the site of infection cause localized infections. Rhinovirus is an example of a virus that causes a localized infection: it infects the epithelial cells of the upper respiratory tract and replicates there. Similarly, papillomavirus strains that infect the skin replicate locally in the epidermis. On the other hand, viruses that initiate infection through one organ but then spread to other sites within the body cause systemic infections. These viruses infect cells within the initial organ, where they either replicate locally before spread or use infected local cells to travel to other locations within the body.

Virions spread to other organs through one of two ways. In hematogenous spread, viruses spread to target organs using the bloodstream. This can occur through direct injection into the blood, as would happen with animal or insect bites, or it can occur from virions entering the interstitial fluid that bathes all our cells within the tissues. This interstitial fluid, or lymph, is collected in lymphatic vessels that lead back to lymph nodes (Fig. 5.8). Immune system cells filter the lymph within the lymph nodes, but high virion concentrations can mean that some escape these cells and continue within the lymph, which is eventually returned to the bloodstream. Viremia is the term used to describe the presence of virus within the bloodstream. Since blood circulates throughout the entire body, viruses can use the bloodstream to gain access to their cellular targets in organs other than the ones through which they entered the body. Primary viremia indicates the first time that the virus is found in the bloodstream. After replicating within the target organ, additional virions may enter the bloodstream. This is known as a secondary viremia.

In neurotropic spread, viruses spread through the body using neurons. Viruses rarely infect neurons directly because it is difficult for viruses to directly access these cells. Most often, viruses replicate in cells at the local site of infection and then infect neurons located nearby. Several herpesviruses spread in this manner, infecting and replicating within the local epithelium until sufficient virions are present to infect nerves associated with the tissue.

Herpesviruses are dsDNA viruses and must gain entry into the cell’s nucleus for replication. However, the nucleus of a neuron, can be quite a distance from the axon terminal where the virus initiated attachment to the cell. As mentioned in Chapter 4, “Virus Replication,” herpesviruses have evolved mechanisms to overcome this challenge. Viral proteins bind to dynein, a host cell protein that “walks” vesicles of cargo along microtubules to the nucleus. At the nucleus, the capsid docks at a nuclear pore and its viral DNA is transported into the nucleus for replication or latency.

Viruses first infect neurons of the peripheral nervous system, which can be used to access the central nervous system. Certain viruses can cause devastating results if they reach the central nervous system. For instance, death occurs within days of the rabies virus reaching the brain.

5.3. Portals of Virus Exit

In order to persist within a population, a virus must spread from an infected host to a susceptible host. The shedding of virus refers to the release of infectious virions from the host. During localized infections, the virus is shed from the primary site of infection. Viruses that infect the skin are spread through skin-to-skin contact, and respiratory viruses are shed within respiratory secretions, passed along through a cough or sneeze to a new, susceptible host. Gastrointestinal viruses are shed within aerosolized vomit or diarrhea, potentially contaminating food or water that could infect a subsequent individual. Viruses that replicate in the lungs, nasal cavity, or salivary glands can be shed in saliva, and viruses such as HIV and herpesviruses can replicate within genital compartments and be shed in sem*n or vagin*l secretions.

Viremia is a common occurrence of infection with several viruses, including HIV and hepatitis. Consequently, these viruses can be transmitted through blood. Viruria, the presence of virus within the urine, occurs with several systemic viral infections, including measles and mumps. Other viruses replicate within and are shed by cells of the urogenital tract, such as JC polyomavirus (JCPyV) and BK polyomavirus. Some viruses that cause severe disease are transmitted to people through aerosolized virions found in rodent urine or droppings (see In-Depth Look).

Although viruses cannot replicate independently outside a host, virions can remain infectious outside the body. The stability of virions within the environment, however, depends upon several factors, both of the environment and of the virion itself (Table 5.3). The biochemical characteristics of the virion and its contents, including the type of nucleic acid, the sensitivity of viral proteins to pH changes, and the presence or absence of a lipid envelope, play a role in the sensitivity of the virion to inactivation by environmental factors. Temperature, humidity, moisture content, sunlight, pH, and the presence of organic matter all affect the inactivation of virions within the environment.

Viruses within contaminated feces or urine can be transmitted via the fecal–oral route or shed within feces and urine and then aerosolized and inhaled. The neutral pH of human waste generally protects virions, and organic matter within feces also buffers the chemical makeup and temperature of the environment. Temperature plays a large part in the persistence of viruses within feces or waste water. Viruses are inactivated within minutes or hours at high temperatures (above 121°F or 50°C), but certain viruses, particularly those that are nonenveloped, can remain infectious for days or months at ambient temperatures.

Airborne viruses can be transmitted within liquid droplets or aerosolized particles that are released when a person sneezes, coughs, speaks, or breathes. Droplets

are larger in size, about 20μm, and therefore tend to only be spread short distances before they succumb to gravity and fall out of the air (Fig. 5.9). At 5μm or less, aerosolized particles remain airborne for much longer periods of time, and the evaporation of liquid from aerosols creates smaller particles that can persist in the air for an extended period of time. Some viruses are better protected from inactivation within droplets. Aerosols, being smaller, generally have greater success in reaching the lower respiratory tract.

For airborne viruses, humidity and temperature often play a role in the persistence of the virion in the environment. It has been shown for several enveloped respiratory viruses, including influenza A virus, measles virus, severe acute respiratory syndrome coronavirus (SARS-CoV), and Middle East respiratory syndrome (MERS)-CoV, that lower temperature and humidity are more conducive to maintaining airborne virions. Virions are inactivated faster at higher temperatures and humidity, and droplets tend to fall out of the air more readily with higher humidity. On the other hand, nonenveloped viruses like rhinovirus and adenovirus remain infectious longer in higher-humidity environments. Organic matter, such as proteins and carbohydrates derived from mucus or aerosolized fecal matter, can also slow the inactivation of viruses within droplets or aerosols. Other viruses are sensitive to light, primarily the ultraviolet component.

5.4. Patterns of Infection

A host typically goes through four stages of disease development when it is infected with a virus (Fig. 5.10). The incubation period is the time between when the virus initially infects the host and when symptoms appear. For example, the incubation period of rhinovirus, a cause of the common cold, tends to be about 1–3 days. The incubation periods of well-known viruses are listed in Table 5.4.

The prodromal period occurs after the incubation period and is when symptoms first appear. These tend to be nonspecific, mild symptoms that are not clinically indicative of the type of virus, such as malaise, muscle aches, or a low-grade fever. During this period, however, the virus is replicating quickly within the host. The illness period occurs when specific symptoms of the disease occur. At this point, the virus is multiplying to high levels and the immune system has been activated, but the response takes time. In immunocompetent hosts with functioning immune systems, infected cells will eventually be eliminated and the amount of virus within the host will decline. At this point, the symptoms of the disease subside as the host begins feeling better, having entered into the convalescent period. This period may last for days or months, depending upon the severity of the infection.

The duration of each period of disease can vary depending upon the virulence of the pathogen, the site of infection, and the strength of the host immune system. The period of communicability, when a person is contagious and able to spread the virus to new hosts through virion shedding, will also vary depending upon the virus and can even include part of the incubation period, when symptoms are not yet present, all the way into convalescence (Table 5.4). For example, influenza is contagious from 1day before symptoms arise until about a week after becoming sick. Ebola virus is communicable in breast milk and sem*n for weeks to months during the convalescent period.

The replication and persistence of a virus within a host generally follow one of two different patterns of disease. In an acute infection, the virus replicates rapidly within the host and is spread to other individuals, but the immune system clears the virus, generally within 7–10days. Epidemics are most often caused by viruses that cause acute infection. Some acute infections are inapparent or subclinical, meaning that they produce no symptoms of disease, although the virus still replicates and activates the immune system. Spread to other hosts can still occur with inapparent acute infections. This is different from an unsuccessful infection in which the virus is not able to successfully replicate within a host and does not establish infection, or an abortive infection in which the virus enters susceptible cells but cannot complete replication, usually due to the absence of a protein in the infected cell required for replication.

On the other hand, persistent infections occur when the host immune system is unable to effectively clear the virus, but the virus does not replicate to levels that kill the host. Persistent infections often last for the lifetime of an individual and occur for a variety of reasons. Viral proteins can modulate the immune response, and certain viruses, including HIV, infect immune cells and interfere with their proper functioning. Persistent infections can also result from the production of defective interfering (DI) particles. DI particles are virions that are created during the replication process but contain incomplete or deleted genomes. Although these are defective, they are still released from the cell and act as a sponge to sequester antibodies. Although the mechanism is unknown, they also interfere with the apoptosis of infected cells, which can lead to a persistent state. The production of DI particles is more common in RNA viruses. Finally, certain organs of the body, including the brain, have highly controlled mechanisms to safeguard against cellular damage caused by inflammation. Viral infection of these tissues may lead to persistent infections due to the protection of these cells against apoptosis.

Persistent infections can also result from viral latency, a state in which the virus becomes dormant within host cells. A hallmark of all herpesviruses is that they establish latency. For example, varicella zoster virus causes chickenpox upon primary infection of an individual, but the virus is never completely cleared by the immune system. Instead, it becomes latent in the sensory neurons of the individual. Although the genome persists in the neurons, new virions are not made, and a separate latency gene program is expressed. The virus is largely undetected by the immune system during latency. Later in life, reactivation from latency may occur, at which point the virus switches on a productive infectious cycle. Varicella zoster virus reactivation causes the painful skin rash known as shingles. Latency and reactivation will be discussed in more detail in Chapter 13, “Herpesviruses.”

An unusual variation of persistent infection occurs with slow infections. As the name suggests, these viruses can take years to reach a symptomatic phase (if one ever occurs). HIV establishes a slow infection: without antiretroviral drugs, it takes around 8–10years for an individual to progress to a stage of disease where symptoms are apparent. In this case, symptoms arise as a result of opportunistic infections due to immunosuppression. JCPyV is a very common virus, infecting around half of the general population. JCPyV establishes a slow (and possibly latent) infection in kidney cells. It can reactivate in immunosuppressed individuals to cause progressive multifocal leukoencephalopathy, a brain disease with a high mortality rate that damages the white matter of the brain.

5.5. Epidemiology

Epidemiology is the study of how diseases, including those caused by viruses, spread throughout a population. In fact, the word “epidemiology” means “the study of what falls upon a people.” Epidemiology is the field that studies the spread of noninfectious and infectious diseases, with a goal of identifying how diseases occur and can be controlled. Epidemiologists are the detectives of the public health world: during a disease outbreak, they determine how a disease is being transmitted from person to person and establish control measures that interrupt the continued transmission of the pathogen.

Within a population, infections can be classified based upon their frequency of occurrence. A sporadic disease occurs infrequently and without a consistent pattern. A single case of hantavirus in Nevada would be considered sporadic in nature. Endemic refers to the usual presence of a disease in a population at any given time. It is not necessarily a desired level, but it is the norm for a particular area. For example, during a normal September, 3.5 out of every 1000 people in the United States present with symptoms of rhinovirus. An epidemic occurs when there are clearly more cases of disease in a particular area than are expected during endemic periods. This is also referred to as an outbreak. From August 2014 through January 2015, 1153 people in 49 US states and the District of Columbia contracted enterovirus D68; only small numbers had been previously reported each year. When an epidemic spreads throughout several countries or the world, it is referred to as a pandemic. A major pandemic caused by viruses was the 1918 “Spanish Flu,” which is estimated to have killed 50million people worldwide in the course of months.

5.6. Epidemiological Studies

An orderly examination of all the facts surrounding an outbreak is required for epidemiologists to accurately investigate the variables within the chain of infection. Epidemiologists also determine the morbidity (rate of illness) and mortality (rate of death) associated with an illness. The incidence of the disease refers to the number of new cases within a population during a specified time, while the prevalence of a disease refers to the total number of individuals with the disease at that time. For example, the US CDC reports that in 2012, the incidence of HIV in the United States was 18.4 cases per 100,000 people, while the prevalence of the disease was 342.1 cases per 100,000. In other words, 18.4 new cases were diagnosed and 342.1 people were living with HIV in 2012, per 100,000 people.

Before counting cases, however, an epidemiologist must determine what qualifies as a case of the illness. Epidemiological studies rely upon a case definition to determine whether or not a person has a particular disease. The case definition is a set of clinical and laboratory criteria that rely upon the symptoms the person presents with and the results of virus-specific blood tests. For a specific outbreak, limitations on the time and location may also be included within the case definition. Nationally and internationally, use of a standard case definition allows for proper diagnosis and also ensures comparability among different hospitals and locations. Classification of results can categorize the case as suspected, probable, or confirmed (Fig. 5.14).

Case definitions can change over time and often do so as more information about the illness becomes available. Case definitions can also possess “loose” or “strict” requirements. A sensitive (“loose”) case definition is used for containment of viruses with potentially serious effects upon public health; this type of case definition is not very specific, in an attempt to include all possible cases of the virus even if other viral infections may fall within the case definition. When trying to determine the specific cause of an outbreak, however, epidemiologists employ a strict case definition that will only confirm those infected with the particular pathogen. For instance, an epidemiologist studying the cause of a Norwalk virus outbreak, which is a fecal–oral disease that causes diarrhea, will not want to include diarrhea as the only requirement—many other infectious and noninfectious diseases also cause diarrhea as a symptom. Loose and strict case definitions may overestimate and underestimate the total number of cases, respectively, but are necessary for containing serious viral outbreaks or determining the definitive source of an infection.

In the United States and in many countries around the world, state and national public health departments must be notified when a patient is diagnosed with certain infectious and noninfectious conditions. This surveillance system is used to monitor disease trends, identify populations at high risk, formulate control measures, and create public health policies. The list of notifiable viral diseases is listed in Table 5.6.

Just like reporters, epidemiological studies seek to identify the 5W’s of the epidemiological story: What (the agent), Who (the person infected), Where (the location), When (the time), and Why/How (the causes and modes of transmission). These are also known as epidemiological variables. Epidemiological studies are divided into descriptive and analytic studies. Both types of study start with the agent, defined by the case definition, and identify the who, where, and when (host, location, and time). In addition, analytic studies try to determine the cause and transmission.

Time refers not only to the hour and minute of the day, but also to the season and time of year. Some diseases occur more frequently during a particular time of the year; for example, influenza viruses peak during the winter months (Fig. 5.15), when drier weather supports the increased dissemination of aerosolized particles in the environment and people congregate inside more often. In contrast, the incidence of diseases transmitted by mosquitoes, such as West Nile virus or eastern equine encephalitis virus, is higher at the end of the summer when mosquito populations are highest. Other viral infections show no association with season or time of year, such as hepatitis B virus, HIV, or HPV. Determining the typical yearly or seasonal pattern of viral infections is important in creating a baseline that can be used to compare future occurrence of the disease or monitor the effectiveness of control measures. Graphing the time of the incident versus the occurrence of symptoms can also be useful in assessing the incubation period of the virus, which can be helpful in identifying the specific virus.

Place refers to the local location of the case as well as the larger geographic location. A gastrointestinal virus outbreak may occur at a restaurant (the place), suggesting that the food may have been involved in the transmission of the virus. A person presenting with neurological manifestations in a rural location may suggest different viruses than in a big city—for example, rabies transmission is much more likely in areas where wildlife is prevalent. It would also be important to note that a patient presenting with hepatitis recently traveled to a remote location in South America. This also highlights that, because of viral incubation periods, the location and timing of symptoms does not always correlate with when infection occurred (Fig. 5.16).

Details concerning the person (host) can affect the chain of infection and are important to consider in epidemiological studies. Almost every health-related event varies with age because it is a factor in exposure, immune status, and physiological response. For example, older individuals are much more likely to reactivate varicella zoster virus, leading to shingles, whereas children are most likely to show symptoms of the “childhood diseases” to which adults have already become immune. Influenza A virus causes higher morbidity and mortality rates in young children and the elderly.

Many other personal attributes can contribute to infection. The sex of the individual can sometimes be a consideration—cervical cancer caused by HPV will not occur in men—and being part of different genetic, cultural, or social groups can contribute to the exposure of an individual to a particular virus. The socioeconomic status of an individual (income, education, and occupation) can also play a role in exposure to the virus and access to medical care.

Descriptive studies are effective in chronicling patterns and developing hypotheses as to the cause of an illness or outbreak. In addition to the what, when, where, and who epidemiological variables, analytic studies are also concerned with the why/how of the illness. A hallmark of analytic studies is the presence of a comparison (control) group that can be used to generate baseline data to which the outbreak or illness can be compared. With a comparison group, statistical analyses can be performed to determine a cause with good certainty.

Epidemiological studies fall into two general categories: experimental and observational. Experimental studies are planned, controlled studies; a clinical trial to test new vaccines that enrolls participants into the study, randomly assigns them into one of three groups (vaccine A, vaccine B, or placebo), and then gathers data is an experimental study. As the name implies, observational studies involve the observation of subjects and subsequent recording of data. In comparison to experimental studies, the epidemiologist does not have any influence over what exposure the participant receives. Observational studies are more common in epidemiology than experimental studies.

Observational studies fall into three categories: cohort studies, case-control studies, and cross-sectional studies. Cohort studies are similar to experimental studies in that two groups are compared in real time, except that, being an observational study, the epidemiologists do not assign participants to cohorts, or groups. Instead, they allow the natural course of things to proceed, tracking whether or not the two cohorts have different results. An example of a cohort study would be to observe consistent users of hookah pipes, a water pipe used to smoke flavored tobacco. One cohort of participants uses individual disposable plastic mouthpieces on their pipes, while the other group uses the attached metal mouthpiece. This study might observe whether HSV-1 transmission is more common in those using the metal, shared mouthpieces compared to those who each have their own disposable plastic mouthpiece. If this were an experimental study, the epidemiologists would have assigned each participant to a specific group, either the group that uses the shared mouthpiece or the disposable plastic mouthpiece.

A second type of observational study, the case-control study, is always retrospective, meaning that it analyzes past events. A case-control study happens after an event (for example, a viral outbreak) has occurred. Being that the outbreak has already happened, a control group is assembled retroactively with a group of similar people in a similar place as the outbreak to see if, in fact, the “outbreak” was different from the norm. An example of a case-control study would involve the infection of several people with hepatitis C virus at a tattoo parlor. After noting that all the case patients received their tattoos from one tattoo artist, epidemiologists retroactively enrolled a group of people that received tattoos from the other tattoo artist at the parlor. The control group allows them to have a baseline group to determine the typical infection rate. In this case, it was determined that one of the tattoo artists, but not the other, was improperly sterilizing tattoo equipment.

The final type of observational study is the cross-sectional study. In this type of study, data are gathered from a random sample of individuals at one time (a “cross section” of the population) and correlations are made. For example, a cross-sectional study might find that a high proportion of those individuals that have liver scarring have hepatitis C infection. Although the obvious conclusion seems to be that the liver scarring must be caused by the virus, it is also possible that the scarring makes the liver more susceptible to hepatitis C infection, and that is why these individuals have high rates of both liver scarring and hepatitis C. Because the study compares individuals at only one point in time, cause and effect (causation—the how/why) are difficult to determine. Correlation does not equal causation! Therefore, a cross-sectional study is not effective as an analytic study, but they are used routinely for descriptive studies.

Since both the cohort and case-control study have comparison groups, they would be considered analytic studies. Cross-sectional studies often do not have a comparison group as a control and are therefore most often carried out as descriptive studies.

Summary of Key Concepts

Section 5.1 Portals of Virus Entry

• •

  For successful infection to occur, a host must come in contact with the virus (accessibility). The cells must express the cell surface receptors used by the virus (susceptibility), and they must also contain all the internal requirements for viral replication to proceed (permissivity).

• •

  Portals of entry are the locations through which viruses gain entry into the body. Unless delivered directly into the tissues through a mosquito bite or needle, most viruses first interact with the epithelium at the site of entry.

• •

  The majority of viruses enter humans through inhalation into the respiratory tract. The gastrointestinal tract, genital tract, skin, blood, and eyes are also points of entry.

• •

  Acid-labile viruses break down within the low pH of the stomach, while acid-resistant viruses are resistant.

• •

  Some viruses are able to be transmitted through organ transplants or through the placenta to a developing fetus.

Section 5.2 Dissemination Within a Host

• •

  Localized viral infections replicate at the initial site of infection. Systemic infections spread to additional areas throughout the body, generally through hematogenous or neurotropic spread.

Section 5.3 Portals of Virus Exit

• •

  Viruses are shed through portals of exit to infect new hosts. For localized infections, this is generally at the same location through which the virus entered: viruses that infect the skin are spread through skin-to-skin contact, respiratory viruses are shed within respiratory secretions, and gastrointestinal viruses are shed within aerosolized vomit or diarrhea.

• •

  The stability of virions within the environment is dependent upon many factors, including the type of nucleic acid; the presence of a viral envelope; and the sensitivity of virions to pH, humidity, moisture content, and sunlight.

• •

  Respiratory droplets are larger in size (∼20μm) and are spread only short distances before falling to the ground. Smaller aerosolized particles (<5μm) can travel extended distances within the air.

Section 5.4 Patterns of Infection

• •

  Upon infection, a host generally passes through four stages of disease: the incubation period, prodromal period, illness period, and convalescent period.

• •

  Acute infections, during which viruses multiply and spread quickly, are often the cause of epidemics.

• •

  Persistent infections result when the immune system is unable to clear the virus from the body. Persistent infections can also result from viral latency or slow infections.

Section 5.5 Epidemiology

• •

  Epidemiology is the study of how disease occurs in populations. Sporadic diseases occur irregularly, while endemic refers to the normal rate of infection. An epidemic occurs when there are more cases than the normal endemic rate. A pandemic is an epidemic that spreads throughout several countries or the world.

• •

  The epidemiologic triad model represents the factors that bring an agent and susceptible host together in a particular environment. There are many factors that affect each component of the model.

• •

  The chain of infection represents how an agent leaves its reservoir through a portal of exit, is conveyed via a mode of transmission, and enters a susceptible host through a portal of entry. Epidemiologists identify the factors involved at these stages in an effort to institute control measures.

Section 5.6 Epidemiological Studies

• •

  Epidemiological studies are divided into descriptive and analytic studies. Both types use a case definition to confirm persons with a case of the disease. Both also analyze the time, place, and persons involved in the chain of infection, but analytic studies use a comparison group to determine the cause of the outbreak.

• •

  Experimental studies are planned ahead of time and participants are randomly assigned to predetermined groups. Observational studies—which fall into cohort, case-control, and cross-sectional studies—acquire data and track cohorts as they occur within normal situations.

Flash Card Vocabulary

Chapter Review Questions

• 1.

  Make a table listing each portal of entry. What defenses does the host have at each location and how are viruses able to successfully bypass them?

• 2.

  Norwalk virus causes significant morbidity in developed nations, despite that these countries have clean water supplies. How do you think the virus is transmitted, and why it is so successful?

• 3.

  Describe the architecture of the skin and how viruses gain access to each layer and the subcutaneous tissue.

• 4.

  How are vertical and horizontal transmission of viruses different from each other?

• 5.

  Which viruses are capable of initiating transplacental or intrapartum infections?

• 6.

  How do localized infections become systemic infections?

• 7.

  Describe how different factors could affect the inactivation of virions within the environment.

• 8.

  Draw out the stages of infection after a person is infected with a virus.

• 9.

  Your friend walks into class, still sniffling occasionally from a respiratory viral illness. She reassures you that she’s “not infectious anymore.” You have your concerns. Why?

• 10.

  How can persistent infections arise?

• 11.

  Describe the difference between endemic and epidemic.

• 12.

  Make a list of the three major aspects of the epidemiologic triad model and what factors could affect each aspect in promoting infection.

• 13.

  Create a list of control measures that interfere with each variable within the chain of infection.

• 14.

  Design a case-control study that attempts to determine the precise food product that was the cause of a viral gastrointestinal illness.

Further Reading