The invasion and growth of germs in the body. The germs may be bacteria, viruses, yeast, fungi, or other microorganisms. Infections can begin anywhere in the body and may spread all through it. An infection can cause fever and other health problems, depending on where it occurs in the body. When the body?s immune system is strong, it can often fight the germs and cure an infection. Some cancer treatments can weaken the immune system, which may lead to infection.