Answer: germ theory
Explanation: Germ theory of disease is a theory in biology. It says that small organisms (called germs), also known as microbes, cause diseases. Most, but not all diseases are infectious diseases. The germ theory states that small organisms cause a reaction in the body of those who are infected. The body's reaction to infection is called a disease.
Reference: Wikipedia
Photosynthetic bacteria changed the Earth's atmosphere
Food and water supply,habitat space, and competition with other species
A
biology is branch of science which deals with life