evidence: Diseases can have infectious and non-infectious causes. Germ theory holds that certain diseases can be caused by microorganisms. This theory is key to our modern ability to prevent, diagnose, and treat infectious diseases. It explains why things like antibiotics, vaccines, and improvements in sanitation produced public health breakthroughs. While some diseases can be caused by deficiencies in the individual, in keeping with the concept of the terrain theory, such as Vitamin C deficiency, this doesn’t mean that all diseases are caused by imbalances in the body.