Whether living in urban or rural environments, humans tend to perceive the world around them as being shaped by culture and industry more than by natural history. Humans, however, are part of a biological continuum that covers all living species. Charles Darwin's 200th birthday in 2009 could serve to remind us of this. All animals, including humans but also plants, fungi, and bacteria, share the same basic biochemical principles of metabolism, reproduction, and development. Most pathogens can infect more than one host species, including humans. In 1964, veterinary epidemiologist Calvin Schwabe coined the term "one medicine" to capture the interrelatedness between animal and human health, and the medical realities of preventing and controlling zoonotic diseases or "zoonoses" -diseases that are communicable between animals and humans. One medicine signaled the recognition of the risks that zoonotic diseases pose to people, their food supplies, and their economies. Given the interrelatedness of human, animal, and ecosystem health, the rationale for some form of coordinated policy and action among agencies responsible for public health, medical science, and veterinary services is quite intuitive. Later, the term "one health" came into use, and later still, the broader concept of "one world one health," which is today used to represent the inextricable links among human and animal health and the health of the ecosystems they inhabit.