The germ theory of disease states that some diseases are caused by microorganisms. These small organisms, too small to see without magnification, invade humans, animals, and other living hosts. Their growth and reproduction within their hosts can cause a disease. "Germ" may refer to a virus, bacterium, protist, fungus, or prion. Microorganisms that cause disease are called pathogens, and the diseases they cause are called infectious diseases. Even when a pathogen is the principal cause of a disease, environmental and hereditary factors often influence the severity of the disease, and whether a particular host individual becomes infected when exposed to the pathogen. The germ theory was proposed in the mid-16th century and gained widespread credence when substantiated by scientific discoveries of the 17th through the late 19th century. It supplanted earlier explanations for disease, such as Galen's miasma theory.