Disease cause is a term used to describe the source of an illness. The three most common causes of disease are microbes, pathogens, and germs. Microbes are tiny organisms that can cause infection and disease, such as bacteria and viruses. Pathogens are microorganisms that can cause disease in humans, animals, and plants. Germs are microscopic organisms that can cause infection and disease. All three of these causes of disease can be spread through contact with infected people, animals, or objects.