Both bacteria and viruses and the most important causes of disease. Bacteria are simple plant-like organisms that can divide very quickly. They cause many common infections such as boils and acne.
Viruses are very much smaller and technically they are not alive at all. They can take over the functioning of an infected cell and turn it into a factory producing millions more viruses. Viruses are responsible for many common diseases such as colds and influenza.
The diagram below shows us how a virus invades a cell: (1) they shed their outer layer (2) and take over the genetic material in the host cell in order to reproduce themselves (3). They being to construct protein coats around the new virus (4) and eventually burst out of the host cell (5) to leave it in an envelope (6) ready to infect new cells.