In the worst-case scenario, the two countries could have a total of 21,000 cases of Ebola by Sept. 30 and 1.4 million cases by Jan. 20 if the disease keeps spreading without effective methods to contain it. These figures take into account the fact that many cases go undetected, and estimate that there are actually 2.5 times as many as reported.
In the best-case model, the epidemic in both countries would be “almost ended” by Jan. 20, the report said. Success would require conducting safe funerals at which no one touches the bodies, and treating 70 percent of patients in settings that reduce the risk of transmission. The report said the proportion of patients now in such settings was about 18 percent in Liberia and 40 percent in Sierra Leone.
The caseload projections are based on data from August, but Dr. Thomas R. Frieden, the C.D.C. director, said the situation appeared to have improved since then because more aid had begun to reach the region.
“My gut feeling is, the actions we’re taking now are going to make that worst-case scenario not come to pass,” Dr. Frieden said in a telephone interview. “But it’s important to understand that it could happen.”
Outside experts said the modeling figures were in line with estimates by others in the field.
“It’s a nice job,” said Ira Longini, a professor of biostatistics at the University of Florida who has also done computer modeling of the epidemic. “It summarizes the extent of the problem and what has to happen to deal with it.”
Bryan Lewis, an epidemiologist at the Virginia Bioinformatics Institute at Virginia Tech, agreed that the estimates were reasonable, perhaps even a bit low compared with those generated by other models. He said that if some of the latest data from the World Health Organization is plugged into the C.D.C. model, “the very large numbers of estimated cases are, unfortunately, even larger.”
The current official case count is 5,843, including 2,803 deaths, according to the W.H.O.
The C.D.C. estimates omit Guinea, which has been hit hard, because the epidemic struck in waves that could not be modeled.
The W.H.O. published its own revised estimates of the outbreak on Monday, predicting more than 20,000 cases by Nov. 2 if control does not improve. That figure is more conservative than the one from the C.D.C., but the W.H.O. report also noted that many cases were unreported and said that without effective help, the three most affected countries would soon be reporting thousands of cases and deaths per week. It said its projections were similar to those from the C.D.C.
The W.H.O. report also raised, for the first time, the possibility that the disease would not be stopped but could become endemic in West Africa, meaning that it could become a constant presence there.
President Obama’s promise last week to send 3,000 military personnel to Liberia and to build 17 hospitals there, each with 100 beds, was part of the solution, Dr. Frieden said. But it was not clear when those hospitals would be ready, or who would staff them.
Dr. Frieden said the Defense Department had already delivered parts of a 25-bed unit that would soon be set up to treat health workers who become infected, a safety measure he said was important to help encourage health professionals to volunteer. He said that more aid groups were also arriving in the region to set up treatment centers, and that a “surge” of help would “break the back of the epidemic.”
Dr. Jack Chow, a professor of global health at Carnegie Mellon University and a former W.H.O. official, said, “The surge only becomes realized when those beds are up and operating and the workers are delivering care.”
He added, “If even the medium case comes to pass, with, say, 700,000 cases by January, the epidemic will quickly overwhelm the capabilities that the U.S. plans to send.”
The W.H.O. reported that a new center had just opened in Monrovia, the Liberian capital, with 120 beds for treatment and 30 for triage. Patients were already lined up at the door.
The report from the C.D.C. acknowledged that case counts were rising faster than hospital beds could be provided. It said that in the meantime, different types of treatment would be used, based in homes or community centers, with relatives and others being given protective gear to help prevent the disease from spreading.
The United States government is also sending 400,000 kits containing gloves and disinfectant to Liberia to help families take care of patients at home.
At least one aid group in Liberia is already shifting its focus to teaching people about home care and providing materials to help because there are not enough hospital beds for the sick. Ken Isaacs, a vice president of the group, Samaritan’s Purse, said, “I believe inevitably this is going to move into people’s houses, and the notion of home-based care has to play a more prominent role.”
“Where are they going to go?” he said.
Though providing home-care kits may seem like a pragmatic approach, some public health authorities said they were no substitute for beds in isolation or containment wards.
But Dr. Frieden said that home care had been used to help stamp out smallpox in Africa in the 1960s. The caregivers were often people who had survived smallpox themselves and were immune to it. Some experts have suggested that Ebola survivors might also be employed to care for the sick.
Dr. D. A. Henderson, who led the W.H.O.’s smallpox eradication program, said that local people had been paid to help in the campaign.
“We recruited a lot of people to stand guard at huts with smallpox,” said Dr. Henderson, a professor at the Johns Hopkins Bloomberg School of Public Health and the University of Pittsburgh. “The important thing was to know they got paid.”
He added: “We gave money and food to families who had smallpox so they didn’t have to go out and beg, and they didn’t have to go to the market and potentially infect people. What can you do? If you don’t have food, you’ve got to leave the house and go out. Money can play a useful role.”