Viral, bacterial, and parasitic diseases have accompanied humankind since the earliest times and have had more impact on history than any other single factor.

Other than the direct toll inflicted by infectious diseases, numerous studies have established a relationship between microorganisms and chronic conditions such as atherosclerosis, neurologic disorders, cancer, and obesity. The link between microorganisms and increasing numbers of diseases never before envisioned as having microbial etiology opens fascinating scientific, medical, and public health perspectives.