What is a virus?

Viruses are some of the smallest known organisms, and can cause serious disease in humans.A virus consists of a fragment of genetic material inside a protective protein coat. For a virus to survive and replicate, it must enter and infect a host cell. The virus takes over the host cell and replicates inside the cell; this causes damage to the host and leads to symptoms of disease. Viruses are responsible for a variety of human diseases including: flu, common cold, chicken pox and AIDS.