Avirus is an infectious agent that can only replicate within a host organism.Viruses can infect a variety of living organisms, including bacteria, plants,and animals. Viruses are so small that a microscope is necessary to visualizethem, and they have a very simple structure. When a virus particle isindependent from its host, it consists of a viral genome, or genetic material,contained within a protein shell called a capsid. In some viruses, the proteinshell is enclosed in a membrane called an envelope. Viral genomes are verydiverse, since they can be DNA or RNA, single- or double-stranded, linear orcircular, and vary in length and in the number of DNA or RNA molecules.
Theviral replication process begins when a virus infects its host by attaching tothe host cell and penetrating the cell wall or membrane. The virus's genome isuncoated from the protein and injected into the host cell. Then the viralgenome hijacks the host cell's machinery, forcing it to replicate the viralgenome and produce viral proteins to make new capsids. Next, the viralparticles are assembled into new viruses. The new viruses burst out of the hostcell during a process called lysis, which kills the host cell. Some virusestake a portion of the host's membrane during the lysis process to form anenvelope around the capsid.
Following viral replication, the new viruses may goon to infect new hosts. Many viruses cause diseases in humans, such asinfluenza, chicken pox, AIDS, the common cold, and rabies. The primary way toprevent viral infections is vaccination, which administers a vaccine made ofinactive viral particles to an unaffected individual, in order to increase the individual'simmunity to the disease.
