The number of Ebola cases in West Africa is rapidly increasing and the crisis is only going to get worse, a spokesperson for the U.S. Centers for Disease Control and Prevention said Tuesday. Experts fear the ongoing effort to contain the virus, which has killed more than half of the people who have become infected during the current outbreak, could be a losing battle.

“It is the world’s first Ebola epidemic, and it’s spiraling out of control,” Tom Frieden, director of the Centers for Disease Control and Prevention, said Tuesday on “CBS This Morning.” “There is still a window of opportunity to tamp it down, but that window is closing. We really have to act now.”

A fourth American, a missionary doctor treating obstetrics patients at a hospital in Monrovia, Liberia, tested positive for Ebola on Tuesday, according to the Washington Post. The doctor, who has not been named, was not directly working with Ebola patients, according to Enternal Love Winning Africa, his employer.

Three other U.S. citizens working in West Africa have contracted Ebola; two of them were taken to Emory University Hospital in Atlanta, Georgia, where they were given an experimental treatment and recovered. The third, a Liberian American who became infected after arriving in Nigeria, has died.

“It’s bad now, much worse than the numbers show,” Frieden told CNN. “It’s going to get even worse in the very near future.”

Experts are concerned that the Ebola outbreak could also have a devastating impact on the fragile economies of Liberia, Sierra Leone and Guinea, which have fared worst from the disease, as the death tolls continue to rise there. So far, more than 1,500 people have died from the virus.