Viruses are sub-cellular agents of infection that must utilize the cellular machinery of bacteria, plants or animals in order to reproduce. Composed of a single strand of genetic material (DNA or RNA) encased in a protein capsid, a virus is too small to be seen by standard light microscopy; indeed, most are less than one hundredth the size of a bacterium.
Specific proteins on the viral capsid attach to receptors on the host cell; this attachment process is essential to viral infectivity and explains why viruses may only infect the cells of certain species or may only infect certain cells or tissues within a given host species. While the infecting virus triggers an immune response in the host, some are capable of suppressing that response by infecting and killing cells that control immunity (e.g. HIV attacks lymphocytes). In addition, while most infected cells are destroyed by viral replication, some viruses enter a latent phase within cells, reactivating in the future to produce chronic or relapsing infections. Many viruses use specific carriers (known as vectors) such as mosquitoes, ticks, bats and rodents that transmit the virus to a susceptible host while others are spread between individuals via blood contact or through respiratory, intestinal or sexual secretions. Of special concern is the fact that mutations within the viral genome may allow viruses to skip from one host (e.g. birds, swine, monkeys) to another (e.g. humans), unleashing pandemics.
Many common human infections are produced by viruses; these include the common cold, influenza, mononucleosis, herpes infections (including shingles), viral hepatitis (A, B, C and others), HIV, viral gastroenteritis, conjunctivitis, viral pneumonia, encephalitis, viral meningitis and viral infections of the heart, including pericarditis and myocarditis. While viruses do not respond to antibiotics, specific antiviral agents may control (though not cure) chronic disease (such as HIV, Hepatitis B and Hepatitis C) or may modify the severity of acute infection (as in influenza and herpes infections). However, in most viral infections, treatment is, for now, purely symptomatic and supportive. On the other hand, vaccines are capable of preventing some viral infections (e.g. herpes simplex, measles, mumps, rubella, varicella, Hepatitis B) or reducing the severity of an acute infection (e.g. influenza). Beyond the acute or chronic illness that they produce, some viral infections (such and Hepatitis C and certain strains of herpes simplex) are known to be precursors of malignancy. Finally, many researchers suspect that viruses play a role in the pathogenesis of chronic illnesses such as multiple sclerosis and autoimmune disorders.
Update (2017) - Since this post was written, drugs have been developed that cure Hepatitis C; no doubt, other cures for viral infections will follow. R. Folzenlogen
