Infectious Diseases (human)
Infectious diseases are those communicable illnesses that are caused by germs, most commonly by either bacteria or viruses. Plants and animals are subject to infectious diseases, but the term is also used to refer to a branch of medicine. This article focuses on human infectious disease.
A prion is the simplest substance that can cause an infectious disease. A single protein, a prion is a particle that, when ingested, cause the susceptible host to induce additional prions inside the body. These abnormally folded proteins cause malfunctions in normal physiology and so produce disease.