What is a Virus?

Viruses are the smallest known forms of life. Hepatitis C is only 50 nanometers across (a nanometer is one billionth of a meter). If you could put 200,000 hepatitis C viruses end to end, they would be only a single centimeter long.

Viruses use host cells—bacteria, plant cells, or even human body cells—to reproduce themselves. Viruses insert their genetic material into a cell and transform it into a "factory" for making viruses. Sooner or later this kills the infected cell—causing disease.

Viruses usually infect only specific types of cells (frequently in just a


Adenovirus: A typical virus.

single species)—like human lung cells (pneumonia) or human liver cells (hepatitis)—so different kinds of viruses cause different kinds of disease.

Viruses are responsible for a wide range of diseases in humans, ranging from the common cold and the flu to

smallpox, AIDS, and hepatitis.

Human beings and other organisms have developed specialized defenses to protect us from viruses. Chief among these defenses is the immune system, which can disable and kill viruses.

Hepatitis C is a one of a special set of viruses, called RNA viruses, which can outmanuever the human immune system. They do this by mutating rapidly, often evolving faster than the immune system can develop an effective response to them. Infections by RNA viruses like Hepatitis C are hard to beat—and can be very dangerous.