Microbiology is a broad category which includes virology, mycology, parasitology, bacteriology, immunology and other branches. A microbiologist is a specialist in microbiology and these related topics. Microbiology is the study of microscopic organisms, which are defined as any living organism that is either a single cell, a cell cluster, or has no cells at all. This includes eukaryotes, such as fungi and protists, and prokaryotes. Viruses and prions, though not strictly classed as living organisms, are also studied. Microbiology typically includes the study of the immune system, or immunology. 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".