After killing hundreds of millions of people in the 20th century alone and ravaging human populations for the past 6,000 years, the vicious smallpox virus was officially eradicated in the 1970s. The vaccine that brought an end to the disease was called vaccinia, but how this miraculous product was developed has remained a mystery. One U.S. scientist is honing in on solving that mystery, however, and appears to have made an inadvertent discovery in the process.

According to historical records, vaccinia might have originated from the horse, said José Esparza, MD, PhD, who recently retired from the Bill & Melinda Gates Foundation in Seattle, Washington, where he was senior advisor on global health and vaccines. That's right: The horse could be responsible for saving us from the only human infectious disease ever to be eradicated in the history of medicine.

In the late 18th century, British scientists discovered they could protect people from smallpox by inoculating them with the “materials” found in cowpox lesions in cattle. In fact, that’s where the word “vaccine” (from the Latin for cow, vacca) comes from. And, thus, began the “enormous effort” to vaccinate children worldwide against smallpox throughout the 19th century, Esparza said.

However, historical records indicate that people also used horsepox cases to obtain the “preservative against smallpox,” especially in France, Esparza said. “What we could call ‘equination’ was practiced side by side with vaccination,” he said.

In other words, the horsepox virus infected horses, causing fever and pox bumps,