Bacteria was one of the first life forms on Earth. They are single-celled microorganisms (that means really small living things) that either live by itself (get its own food, move around on its own) or as a parasite (is dependent on another living thing to stay alive).

A virus is a microorganism (even smaller than a bacteria) that cannot grow or live outside another living cell. Viruses invade living cells and uses all the cell’s resources (food, energy). Viruses also cannot reproduce (they make clones or copies of themselves). The viruses keep making more and more copies until the cell that was invaded bursts. Then the copied viruses go out and try and find a new cell to invade.