Viruses are small infectious agents of genetic material (either DNA or RNA) contained in a protective protein covering called a capsid. Viruses cannot survive in a host on their own; rather they must use the host cell's metabolic machinery to survive. Viruses are usually species or organ specific.
Virus particles use proteins in their capsid to attach to host cells through receptor molecules on the host cell surface. Once inside host cells, viruses take over the host cells' metabolic machinery to multiply. Viruses may lie dormant for long periods of time, establish a persistent long-term infection without damaging the host cell, or kill host cells when they release daughter viruses.
Viruses may be transmitted to humans by direct transmission from person-to-person, as in contact with airborne droplets from a sneeze or cough, sexual contact, hand-to-eye or mouth contact, mouth-to-mouth contact, or via contaminated blood. Sometimes an insect vector is involved in transmission to humans, as in Yellow Fever or West Nile virus.
Examples of common viral diseases include the common cold, HIV, influenza, chickenpox (varicella virus), hepatitis viruses, infectious mononucleosis (Epstein Barr virus), mumps, and measles. In addition, some viral agents (such as smallpox) can be aerosolized and could be developed as bioterrorism agents.
Antibiotics used to treat bacteria are not effective for treating viruses. A number of antiviral agents have been developed to specifically treat certain viruses, most notably HIV, herpes viruses, and hepatitis B and C viruses. The primary means of treatment of viral infections is prevention through immunization. Vaccines are available for the prevention of several significant human diseases, including poliomyelitis, measles, mumps, rubella, tetanus, hepatitis B, influenza, rabies, and smallpox. Vaccines are made from live attenuated (virulence is radically diminished) or killed virus.