And then we have viruses. Strictly speaking, a virus is not a living entity. A virus is a piece of microscopic dust - a strip of either DNA or RNA covered with a layer of proteins. That's how they reproduce themselves. So the cell stops doing what it was doing and starts writing letters, usually destroying itself in the process.
Every virus likes a certain kind of cell, depending on the layer of proteins that surrounds it.
The virus that causes Parkinson's Disease likes nerve cells, while the virus that causes warts likes skin cells. The kind of cell a virus likes, and how the virus affects the other functions of a cell, govern how it affects the organism as a whole. A virus can cause a cold, or it can give you polio. Surprisingly enough, she was. If Mr. Burchell had been this good, she might not have forgotten this stuff in the first place.