The germ theory of disease is a perspective which asserts:

Answer: Diseases are caused by a biological agent.

Germ theory of disease, the currently accepted theory of disease, says that many diseases are caused by microorganisms. These microorganisms are biological agents, too small to see without a microscope, they invade humans and animals to be able to grow and reproduce. These "Germs" can be protists, bacteria, fungi, viruses, prions, or viroids.