Infection is what happens when germs (also called microbes or microorganisms) enter the body, multiply, and cause illness. The main types of germs are bacteria, viruses, protozoa (parasites), and fungal organisms (also called fungi).Infections in people who have cancer or are getting cancer treatment can be more serious than those in other people. They can also be harder to treat. If you have cancer, it is important to find infections early and treat them quickly -- before they get worse and spread.The good news is that by learning more about them, you and your family may be able to help prevent problems that infections can cause.