A bacterium is a single-celled organism that can spread very quickly, because a bacterium can split itself in two and thus continue to live as two organisms. not all bacteria are bad: for example, there are bacteria in the intestines that help to digest food.
A virus is much smaller than a bacterium: a virus is a piece of genetic material surrounded by proteins. Viruses settle in body cells and process them until the body cells start making virus proteins themselves. The body cell itself dies, while the virus can multiply itself quickly in this way.
