Usually, the source of the disease enters the body from the outside, and the body is defeated by it, resulting in illness.
The human body has various mechanisms in place to prevent illness. For example, when germs enter the body through an injury, the white blood cells in the blood eat the germs.
Consider a cold. The pathogen of a cold is a virus, and the air is always crawling with viruses.
When you are in good health, you do not catch a cold even if the air is swarming with viruses, but if you are tired, sleep deprived, or have poor nutritional balance, you often catch a cold because of the viruses. In other words, when the body's ability to fight germs is impaired, it becomes ill.
There are also cases in which the cause of the disease is not external but internal, as in the case of cancer, for example. Suddenly, cells in the body produce cancer cells. This is a disease that is difficult to cure because the body is attacking itself.
Thus, there are two types of cases in which a person becomes ill: one is when the source of the disease comes from the outside, and the other is when the source of the disease is originally inside the body.