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.

Things in the outside world can cause sickness too. Poisons in the environment can cause illnesses in people. Not eating the right foods, with their important nutrients, can also cause diseases. But the most common cause of sickness from the outside world is infectious agents. These agents are usually microscopic organisms (living things so small that they can only be seen with the help of microscopes) such as bacteria and viruses, commonly referred to as germs. Bacteria and viruses and other microscopic organisms live in the air, water, and soil that make up our world. They are on the things and people we touch and in the food we eat. Many of them are beneficial: bacteria are needed to make cheese, some bacteria help vegetables grow, and some bacteria clean the environment and enrich the soil by feeding on dead plants and animals. But there are other microscopic organisms that invade the bodies of plants and animals—and people—and cause diseases.