Dr. Sheik Humarr Khan, who contracted Ebola while working in Sierra Leone. REUTERS/Umaru Fofana

Two Americans fighting to curb the Ebola outbreak in West Africa have contracted the disease. Nancy Writebol and Dr. Kent Brantly both tested positive for the virus at the Ebola Consolidated Case Management Center in Monrovia, Liberia.

The center is run by the Christian groups Serving in Mission, or SIM, and the U.S.-based Samaritan’s Purse.

Writebol worked sanitizing people and equipment coming from the hospital’s Ebola ward. She was working in Liberia with her husband David, who says he is unable to see her. Brantly, 33, was the medical director for the Ebola treatment center.

Ken Isaacs of Samaritan’s Purse told the Associated Press Brantly "was in a stable but very serious condition and Ms. Writebol was in stable and serious condition.”

The virus has killed 672 and infected more than 1,000 people across West Africa since it first broke out in March. The mortality rate for the outbreak is 60 percent, but death rates for Ebola can be as high as 90 percent. Survival rates are much higher when the virus is caught early on.

According to their biographies on the SIM website, David and Nancy Writebol joined SIM in 2013 after 14 years of working with orphans and disadvantaged children. They have two children. Brantly is also married with two children and AFP reports he has worked with SIM’s mission in Liberia for less than a year and on Ebola specifically since June.

Liberia announced Sunday it was closing all but a handful of essential border crossings to try to contain any potential outbreak. In neighboring Sierra Leone, a woman who was forcefully taken from a treatment center by her family died from Ebola. Sierra Leone officials worried she could spread the virus in her densely populated Freetown neighborhood, and have since quarantined part of it and brought her parents in for blood tests.

One of Liberia’s top Ebola doctors died Saturday after contracting the virus. Another top doctor in the region, Dr. Sheik Humarr Khan working with Doctors Without Borders in Sierra Leone, has also contracted the disease. He’s being treated at a facility there and is “responding to treatment,” the Sierra Leone Ministry of Health said.

Nigerian authorities have also taken precautionary measures to isolate anyone who came in contact with Patrick Sawyer, a Liberian man who died while on a business trip in Nigeria. The hospital at which he was treated has been shut down for a week and authorities are monitoring 59 people with whom he came into contact, including hospital and airport staff and the Nigerian ambassador to Monrovia.