When you get sick, part or all of your body is not working as it should. The cause of sickness can come from inside your body or from the outside world. Diseases that start on the inside are usually inherited in the genes that you receive from your parents, which make up the master plan that determines how your body will grow and run. Abnormal development or functioning of different body systems is the cause of many chronic diseases.

The immune system protects the human body against germs, which are microorganisms that cause sickness and disease. There are four major types of germs— harmful bacteria (pathogens), viruses, fungi, and protozoa. This defense system begins with the skin, which stops germs from getting into your blood or tissues. If germs get into your body, for example through your nose or mouth, white blood cells called phagocytes and lymphocytes attack them. Phagocytes scout out and destroy invaders, and long-living lymphocytes remember the invaders and release chemicals called antibodies to make the body resistant, or immune, to them. White blood cells live in the bloodstream, lymphatic system, and spleen.

Your skin is a wonderful protective barrier that prevents many of the disease-causing germs that you run into each day from entering your body. Only when you have an opening in your skin—like a cut or a scrape—are germs likely to enter there. Most germs enter through your mouth and nose, making their way farther into your body through your respiratory or digestive tracts. But even then, certain chemicals in body tissues and fluids keep many harmful germs from causing problems. When an infection does begin—with the germs multiplying inside your body—your immune system leaps into action to get rid of the foreign organisms. Your white blood cells produce special substances called antibodies that attack and destroy the invaders, helping you to recover.

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 a virus gets inside your body, it attaches itself to a healthy cell and uses the cell’s nucleus to reproduce itself.