Bacteria are single-celled organisms that have the ability to feed themselves and to reproduce. They are found everywhere, including the air, water, and soil. They divide and multiply very quickly, which means that one cell can become 1 million cells in just a few hours! Viruses are microorganisms that are smaller than bacteria, but they cannot grow or reproduce without the help of a separate living cell. Once the virus gets inside your body, it attaches itself to a healthy cell and uses the cell’s nucleus to reproduce itself.