There are many natural and unnatural diseases spreading throughout the world. Some, like rabies, will incubate for a while before striking and most likely killing its carrier. Others, like mummy rot, will slowly but surely decay its victim into nothingness. Some diseases are fairly harmless, but others are very severe.
Protection from disease offers a chance to resist the disease. When a disease is nevertheless contracted, it can, depending on the type of disease, still reduce the speed at which health is lost.