Viruses are the smallest known forms of life, in their simplest manifestions consisting of little more than a small amount of genetic material. Larger and more complex viruses are normally encased in a structure of protein and often a shell of cellular material. Their origins are obscure, but many scientists believe that viruses are among the oldest forms of life. Evolving side by side with every creature on the planet, they have powerfully shaped evolution - even to the point of changing our DNA. It is suspected that many of our genes were originally given to us by viruses.
Lacking the ability to reproduce themselves independently, viruses use host cells - bacteria, plant cells, animal cells, or even human body cells - to carry out their reproductive cycle. After entering an organism, a virus locates and attaches itself to a host cell. Penetrating the cell membrane, the virus inserts its genetic material into the host cell, and transforms the cell into a "factory" for making viruses. In most cases, the virus also shuts down the cell's normal functions to conserve energy for virus production.
Because of this, viruses exact a heavy toll during their reproduction, sooner or later resulting in the death of
their host cells - a process experienced by an organism as 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. Because specific viruses normally infect only specific types of cells (frequently in just a single species) - like human lung cells (pneumonia) or human liver cells (hepatitis) - different symptoms are associated with different types of viruses.