The Ebola virus has the "upper hand" in an outbreak that has killed more than 1,400 people in West Africa, a top American health official said, adding that experts have the tools to stop it.
Dr. Tom Frieden, the director of the U.S. Centers for Disease Control and Prevention, is visiting Liberia, Sierra Leone and Guinea, the three hardest hit countries, this week.
The World Heath Organization estimates that that in those three countries, only one to two doctors are available to treat 100,000 people.
Nigeria has also recorded cases, but officials have expressed optimism that its spread there can be controlled. On Tuesday, Frieden will continue his visit in Liberia, which has the most cases and fatalities.
"Lots of hard work is happening, lots of good things are happening," Frieden told a meeting attended by President Ellen Johnson Sirleaf on Monday. "But the virus still has the upper hand."
The current outbreak is the largest ever and experts have struggled to contain it for a host of reasons: doctors took a long time to identify it, it is happening in a region where people are highly mobile, it has spread to densely populated areas and many people have resisted or hid from treatment. The disease has overwhelmed already struggling health systems in some of the world's poorest countries.
But Frieden expressed optimism that the outbreak can be contained.
"Ebola doesn't spread by mysterious means, we know how it spreads," he said in his remarks, which were broadcast on Liberian TV. "So we have the means to stop it from spreading, but it requires tremendous attention to every detail."
Liberia has resorted to some of the most stringent measures to control the disease, including sealing off an entire slum neighborhood in the capital. Sirleaf has also declared a state of emergency and ordered all her ministers and top government officials to remain in the country or return from any trips.
Late Monday, her office said in a statement that any official who defied that order had been fired. The order was issued a few weeks ago and officials had been given a week to return. The statement did not say how many or who had been fired.
According to the latest WHO tally, the Ebola outbreak has killed 1,427 people of the 2,615 sickened. The U.N. health agency says that 240 health care workers have been infected with Ebola, calling that an unprecedented number. Half of those infected have died.
On Monday, Liberian officials said a top doctor who had received an experimental Ebola drug died.
Dr. Abraham Borbor, the deputy chief medical doctor at Liberia's largest hospital, had received ZMapp after it was given to two Americans who recovered from the disease.
The agency said that the high number of infections among health workers is due to a shortage of protective gear and its improper use and a shortage of staff to treat the tremendous influx of patients.
WHO said the African Union launched an urgent initiative to recruit more health care workers, as anxiety about the disease among the public increases. Patients are shunning some hospitals known to be treating Ebola treatments out of fear, making access to general health care scarce.
The Associated Press contributed to this report.