Update: 5:15: Kent Brantly, one of the two Americans infected by the Ebola virus in Liberia, said in an open letter Friday that "I am growing stronger every day and I want to thank God for his mercy as I have wrestled with this terrible disease."

In the letter posted by Samaritan's Purse, the Christian relief organization that sent him to Liberia, Brantly recalled that "I held the hands of countless individuals as this terrible disease took their lives away from them. I witnessed the horror firsthand, and I can still remember every face and name.

"When I started feeling ill on that Wednesday morning, I immediately isolated myself until the test confirmed my diagnosis three days later," he continued. "When the result was positive, I remember a deep sense of peace that was beyond all understanding. God was reminding me of what He had taught me years ago, that He will give me everything I need to be faithful to Him."

Brantly is recovering at Emory University Hospital in Atlanta, where he was airlifted after falling gravely ill with the disease that is spreading out of control in West Africa. Another missionary from the group, Nancy Writebol, is also in that hospital.

Update: 3:40 p.m.: Nigeria's president declared containment of the Ebola virus a national emergency Friday after the health ministry announced two more confirmed cases, according to the Associated Press. President Goodluck Jonathan also approved the release of $11.7 million to establish more isolation centers, beef up screening at borders and deploy more personnel.

The president asked the public not to spread false information that could lead to "mass hysteria, panic and misdirection," including unsubstantiated suggestions about the "prevention, treatment, cure and spread of the virus," a spokesman said.

The World Health Organization reported four new cases of Ebola in Nigeria Friday, all of them among health care workers and others who had contact with American consultant Patrick Sawyer, a spokesman said.

"They're all contacts of Patrick Sawyer," said Gregory Hartl, a spokesman for the WHO in Geneva. He said all are health care workers, cleaners or others at the hospital where Sawyer was taken after he collapsed in the airport following a flight from Liberia to Nigeria. "We know that he had contact with very few people at the airport," Hartl said.

The images and reports from Liberia, hard hit by a historic Ebola outbreak, are starting to recall scenes from post-apocalyptic films such as "Contagion" and "World War Z."

Sawyer and a nurse two people involved in treating him have died.

The new cases bring the total number of probable or suspected cases in Nigeria to 13, according to the new WHO statistics released Friday. Any spread beyond the transmission chain started by Sawyer in Lagos, a city of 21 million, or Nigeria, a country of perhaps 175 million, would be cause for great concern, but officials are hopeful that has not occurred. Contacts of the people who have fallen ill also are being traced, Hartl said, but Ebola is not contagious until its victims begin to show symptoms, such as high fever.

The new Nigerian cases were reported on Aug. 5 and 6, according to the WHO, when a total of 68 new cases and 29 deaths occurred in the four affected West African countries.

The Nigerian ambassador to the United States, Adebowale Ibidapo Adefuye, said that there have been no Ebola cases outside Lagos. He said that committees of doctors and experts have been set up by the government to respond, and those suspected of having the virus will be quarantined.

“It’s natural to be worried, but we’re confident that the measures being taken by the government will contain it,” Adefuye said  Friday, urging Nigerians not to be panicked by rumors.

According to the WHO, there are now 1,779 cases of Ebola in Guinea, Liberia, Sierra Leone and Nigeria, including 961 deaths.

In other developments:

• The WHO declared the Ebola outbreak in West Africa an international public health emergency Friday, recommending strict monitoring of travelers in the four stricken countries. It said emergency efforts should be put under the direct command of heads of state.

In nations that do not border the affected area, the WHO recommended no general travel ban, but said governments should be prepared to "detect, investigate, and manage Ebola cases," including "the capacity to manage travelers originating from known Ebola-infected areas who arrive at international airports or major land crossing points with unexplained febrile illness." It also urged countries to be prepared to evacuate their citizens, mainly health care workers, who have been exposed to the virus.

• Canadian drugmaker Tekmira Pharmaceuticals said Thursday that the U.S. Food and Drug Administration had removed a “clinical hold” it had placed on the company’s experimental Ebola drug, potentially clearing the way for its use in patients battling the lethal virus.

The company has a $140 million contract with the U.S. government to develop its TKM-Ebola drug, which had shown promising results in treating non-human primates. The drug had been progressing through the FDA approval process until last month, when the agency halted a trial of the drug to seek more information about its safety.

Another experimental drug, ZMapp, manufactured by a San Diego company, was given to U.S. missionaries Kent Brantly and Nancy Writebol who contracted the virus while aiding victims in Liberia. It is too early to determine whether the cocktail of antibodies, never tested on humans, may have helped the pair survive. Both have been brought back to Emory University Hospital in Atlanta where they continue to improve slowly, according to reports.

The government and private companies have said they are ramping up efforts to produce more than the handful of doses currently available, but that effort could take months. The WHO is convening a panel of medical ethicists next week to discuss use of untested treatments in the outbreak, the worst ever.

Brady Dennis contributed to this report.

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