What is a virus? How do viruses work?

A virus is a small infectious agent that replicates only inside the living cells of other organisms. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea.

Viruses are very simple microbes, they can't do much by themselves.  Viruses need a host, another living organism that gives them everything they need to work.  Viruses take any chance they can to find a host.  They get inside the host's cells and take it over.  Viruses use the host cells machinery to make lots of copies, so many that the cell bursts and infects other cells around it!  They can make more viruses very quickly - just one virus can turn into lots of viruses and make you sick!