A second American aid worker in Liberia has tested positive for Ebola , according to the Christian humanitarian group she works for . Nancy Writebol is employed by Serving in Mission , or SIM , in Liberia and was helping the joint SIM/Samaritan 's Purse team that is treating Ebola patients in Monrovia , according to a Samaritan 's Purse statement . Writebol , who serves as SIM 's personnel coordinator , has been living in Monrovia with her husband , David , according to SIM 's website . The Charlotte , North Carolina , residents have been in Liberia since August 2013 , according to the blog Writebols2Liberia . They have two adult children . On Saturday , Samaritan 's Purse announced that American doctor Kent Brantly had become infected . The 33-year-old former Indianapolis resident had been treating Ebola patients in Monrovia and started feeling ill , spokeswoman Melissa Strickland said . Once he started noticing the symptoms last week , Brantly isolated himself . Brantly , the medical director for Samaritan Purse 's Ebola Consolidated Case Management Center in Monrovia , has been in the country since October , Strickland said . `` When the Ebola outbreak hit , he took on responsibilities with our Ebola direct clinical treatment response , but he was serving in a missionary hospital in Liberia prior to his work with Ebola patients , '' she said . Deadliest Ebola outbreak . Health officials say the Ebola outbreak , centered in West Africa , is the deadliest ever . As of July 20 , some 1,093 people in Guinea , Sierra Leone and Liberia are thought to have been infected by Ebola since its symptoms were first observed four months ago , according to the World Health Organization . Testing confirmed the Ebola virus in 786 of those cases ; 442 of those people died . Of the 1,093 confirmed , probable and suspected cases , 660 people have died . There also are fears the virus could spread to Africa 's most populous country , Nigeria . Last week , a Liberian man hospitalized with Ebola in Lagos died , Nigerian Health Minister Onyebuchi Chukwu said . Lagos , the largest city in Nigeria , has a population of more than 20 million . The man arrived at Lagos airport on July 20 and was isolated in a local hospital after showing symptoms associated with the virus . He told officials he had no direct contact with anyone with the virus nor had he attended the burial of anyone who died of Ebola . Another doctor infected . Confirmation of the death in Lagos came after news that a doctor who has played a key role in fighting the Ebola outbreak in Sierra Leone is infected with the disease , according to that country 's Ministry of Health . Dr. Sheik Humarr Khan is being treated by the French aid group Medecins Sans Frontieres -- also known as Doctors Without Borders -- in Kailahun , Sierra Leone , agency spokesman Tim Shenk said . Before falling ill , Khan had been overseeing Ebola treatment and isolation units at Kenema Government Hospital , about 185 miles east of the capital , Freetown . Ebola typically kills 90 % of those infected , but the death rate in this outbreak has dropped to roughly 60 % because of early treatment . What is Ebola , and why does it kill ? Spread by bodily fluids . Officials believe the Ebola outbreak has taken such a strong hold in West Africa because of the proximity of the jungle -- where the virus originated -- to Conakry , Guinea , which has a population of 2 million . Because symptoms do n't immediately appear , the virus can easily spread as people travel around the region . Once infected with the virus , many people die in an average of 10 days as the blood fails to clot and hemorrhaging occurs . The disease is n't contagious until symptoms appear . Symptoms include fever , headache and fatigue . At that point , the Ebola virus is spread via bodily fluids . Get the fast facts on Ebola . Health workers are at especially high risk , because they are in close contact with infected people and their bodily fluids . Adding to the danger , doctors may mistake the initial stages of an Ebola infection for another , milder illness . Deadliest outbreak ever : What you need to know . Is it time to text experimental vaccines ? Why the epidemic is spinning out of control . I survived Ebola .
Aid worker Nancy Writebol becomes second American infected . Dr. Kent Brantly is ill after treating Ebola patients in Liberia last week . The U.S. Centers for Disease Control and Prevention said Thursday that the first case of a person who contracted Ebola outside West Africa has been diagnosed in Texas. A doctor working on the Ebola outbreak in Sierra Leone is also infected .