pathogens: viruses

Viruses are among the smallest organisms known - many times smaller than bacteria. They consist of a fragment of genetic material (DNA strands) inside a protective protein coat.

Viruses can only reproduce inside host cells, and they damage the cell when they do this.

A virus can invade one of our cells and, once inside, take over and make hundreds of thousands of copies of itself. It does this by stealing some of the cell's own precious DNA.

Eventually the virus copies fill the whole host cell and burst it open. The viruses are then passed out via the bloodstream, the airways, or other routes.

A cross-section of a typical flu virus