Answer: Rinderpest

Eradication of an infectious disease is a pretty tall order: to achieve eradicated status, the disease’s prevalence in the host organism has to drop to zero on a global scale (not to be confused with elimination wherein a disease might be declared eliminated in a given country because there have been no new cases of a disease in X number of years).

By that definition, only two diseases have been eradicated: smallpox and the lesser-known rinderpest. Rinderpest, or “cattle plague”, is a viral disease that affects cattle and other ruminants. The disease is closely related to measles and characterized by high fevers, oral erosions, lymphoid necrosis, and a very high mortality rate. Through extensive live-vaccination practices, the world has been rinderpest infection free for over ten years now and the disease is considered completely eradicated.

This makes rinderpest not only the second disease to be eradicated, but the only infectious disease that affects livestock to be eradicated by human effort. If you find it a tad depressing that, despite all our advances in medicine, we’ve only eradicated two diseases, take heart. Although there are still awful diseases afoot that have yet to be eradicated, our progress toward eradicating many diseases is promising. As recently as 1980, there were 52,552 reported cases of polio (and an estimated 400,000 unreported cases) but thanks to global vaccination efforts, the rate of polio has radically declined and in 2014 there were a scant 359 reported cases of polio in the entire world.