A 51-year-old doctor became the third U.S. missionary infected with Ebola as the death toll from the disease in West Africa exceeded all previous outbreaks combined.

The doctor, Rick Sacra, was delivering babies in Monrovia, Liberia's capital, and had traveled between the U.S. and West Africa, the Christian missionary group SIM USA said Wednesday. Sacra was establishing a residency program for family medicine at SIM's hospital in Liberia, the Charlotte, North Carolina-based group said in a statement.

The Ebola outbreak in Liberia, Guinea, Sierra Leone and Nigeria has sickened more than 3,500 people, killing 1,900, more than the 1,590 deaths from the disease over the last 40 years, according to the World Health Organization. The number of cases may exceed 20,000 before the epidemic is controlled, a process that may take six months to nine months, the WHO said.