The existence of microorganisms was hypothesized for many centuries before their actual discovery. They were supposed to live in earth, water, and air, and cause serious diseases. It was not until the 17th century that they were observed and discovered under simple microscopes. The study of microscopic, unicellular, and cell-cluster organisms, including eukaryotes and prokaryotes, is microbiology which typically includes the study of the immune system and viruses. Generally, immune systems interact with pathogenic microbes; these two disciplines often intersect which is why many colleges offer a paired degree such as "Microbiology and Immunology".