When a virus or bacteria (also known generically as a germ) invades your body and reproduces, it normally causes problems. Generally the germ's presence produces some side effect that makes you sick. For example, the strep throat bacteria (Streptococcus) releases a toxin that causes inflammation in your throat. The polio virus releases toxins that destroy nerve cells (often leading to paralysis). Some bacteria are benign or beneficial (for example, we all have millions of bacteria in our intestines and they help digest food), but many are harmful once they get into the body or the bloodstream.
Viral and bacterial infections are by far the most common causes of illness for most people. They cause things like colds, influenza, measles, mumps, malaria, AIDS and so on.
The job of your immune system is to protect your body from these infections. The immune system protects you in three different ways:
- It creates a barrier that prevents bacteria and viruses from entering your body.
- If a bacteria or virus does get into the body, the immune system tries to detect and eliminate it before it can make itself at home and reproduce.
- If the virus or bacteria is able to reproduce and start causing problems, your immune system is in charge of eliminating it.
The immune system also has several other important jobs. For example, your immune system can detect cancer in early stages and eliminate it in many cases.