Viral, bacterial, and parasitic diseases have accompanied humankind since the earliest times and have had more impact on history than any other single factor. Influenza and smallpox devastated cities and ruined empires, each claiming more lives worldwide than all wars combined. Plague killed approximately 200 million, and malaria still claims the life of a child every 30 seconds.
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.
Stein RA, Katz DE. Infections Causing Human Cancer. JAMA. 2008;299(7):837-838. doi:10.1001/jama.299.7.837