What is a virus?

A virus is made of DNA wrapped in a coat made of protein. It is very small, typically nanometres across (one billionth of a metre).

Viruses are different from bacteria as they are smaller and are not structured as cells- they are not made up of a membrane, cytoplasm or nucleus.

It is disputed whether viruses are alive, as the only "living" activity they perform is reproduction, and they can only do this inside cells. This is unlike bacteria, which can reproduce without the help of other cells.

Viruses infect a range of different organisms and many different cell types. Some viruses infect human cells, and some of these cause disease when they do. 

Common viruses that infect humans are: Epstein-Barr virus, EBV (which causes glandular fever) and human papilloma virus, HPV (which can cause cervical cancer).