Human infectious diseases also known as transmissible diseases or communicable diseases are caused by pathogenic microorganisms, such as bacteria, viruses, parasites fungi and prions; the diseases can be spread, directly or indirectly, from one person to another. Zoonotic diseases are infectious diseases of animals that can cause disease when transmitted to humans.

    Microbiology is a scientific discipline studying microscopic organisms, those being unicellular (single cell), multicellular (cell colony), or acellular (lacking cells), which encompasses numerous sub-disciplines including virology, mycology, parasitology, and bacteriology.