Common human diseases caused by viruses include the common cold, the flu, chickenpox and cold sores. Serious diseases such as Ebola and AIDS are also caused by viruses. Many viruses cause little or no disease and are said to be “benign”. The more harmful viruses are described as virulent. Viruses cause different diseases depending on the types of cell that they infect. Some viruses can cause lifelong or chronic infections where the viruses continue to reproduce in the body despite the host’s defence mechanisms.This is common in hepatitis B and hepatitis C virus infections. People chronically infected with this are known as carriers. They serve as important reservoirs of this. If there is a high proportion of carriers in a given population, a disease is said to be endemic.
There are many ways in which viruses spread from host to host but each species of virus uses only one or two. Many viruses that infect plants are carried by organisms; such organisms are called vectors. Some viruses that infect animals, including humans, are also spread by vectors, usually blood-sucking insects. However, direct transmission is more common. And some,such as rotavirus, are spread by contaminated food and water, hands and communal objects and by intimate contact with another infected person, while others are airborne (influenza).Viruses such as HIV, hepatitis B and hepatitis C are often transmitted by transfusion of blood.
It is important to know how each different kind of virus is spread to prevent infections and epidemics.