West Africa is facing the worst outbreak of the deadly Ebola virus in history. Since February, more than 1,200 people have been infected in Liberia, Sierra Leone and Guinea; 672 have died. Liberia has shut its borders, banned funerals and other large gatherings and is considering quarantining certain neighborhoods. One of Liberia’s top doctors died of Ebola over the weekend, and two U.S. healthcare workers in Liberia have also fallen ill. Steve Monroe, deputy director of the U.S. Centers for Disease Control and Prevention, confirmed the infections in a conference call.

Stephan Monroe: “Two American healthcare workers at a hospital in Monrovia, Liberia, have been infected with Ebola virus. One of the healthcare workers, a physician who worked with Ebola patients in the hospital, is symptomatic and is in isolation. The other healthcare worker developed fever, but no other signs of illness. No Ebola cases have been reported in the United States, and the likelihood of this outbreak spreading outside of West Africa is very low.”

A U.S. citizen died of Ebola on Friday in the Nigerian city of Lagos after arriving there by plane from Liberia, marking the first recorded case in Nigeria, Africa’s most populous country. Ebola is highly infectious, has no known cure and can kill up to 90 percent of those afflicted.