A virus is a very, very tiny organism that attacks animals and plants.
Viruses are responsible for many common illnesses, such as the flu.
These instructions tell the cells how to make more cells that are exactly the same. You get this set of instructions from your parents, and every animal and plant has them. The DNA and RNA inside a virus also contain instructions on how to make more viruses. When a virus invades your body, it wants to do just that. The host could be you, your cat, a giraffe or a rose.