Atlanta (CNN) -- The global concern over the spread of Ebola has reached Saudi Arabia, where a man is in critical condition after recently returning from Sierra Leone.
The 40-year-old man has symptoms of a viral hemorrhagic fever, the Saudi Health Ministry said Tuesday. The source of his infection is unknown, but Ebola cannot be ruled out, the ministry said.
Sierra Leone, Guinea and Liberia are at the center of an Ebola outbreak that has already killed more than 800 people.
'This is the biggest and most complex Ebola outbreak in history,' Dr. Tom Frieden, director of the Centers for Disease Control and Prevention, said in a statement.
'It will take many months, and it won't be easy, but Ebola can be stopped,' he said. 'We know what needs to be done.'
The virus has already been confirmed outside of the three main countries.
A Nigerian doctor has been diagnosed with Ebola nearly three weeks after a Liberian-American man with Ebola died after traveling to Lagos, Nigerian officials said Monday.
Nigerian Minister of Health Onyebuchi Chukwu told reporters that the infected physician had been treating Patrick Sawyer, a top government official in the Liberian Ministry of Finance who died of Ebola in a Nigerian hospital July 20.
Eight other people are being quarantined and three are awaiting Ebola test results, the health minister said.
Read more about Patrick Sawyer's death
Ebola transport team speaks to CNN First American Ebola patient comes home Dr. Kent Brantly, right, gets out of an ambulance after arriving at Emory University Hospital in Atlanta on Saturday, August 2. Brantly was infected with the Ebola virus in Africa but was brought back to the U.S. for further treatment. Health officials say the Ebola outbreak centered in West Africa is the deadliest ever. The World Health Organization reports that the outbreak in Liberia, Sierra Leone and Guinea has infected more than 1,300 people and killed more than 700 this year.Photos: Ebola outbreak in West Africa Map: The Ebola outbreak
Map: The Ebola outbreakMeanwhile, the World Health Organization reports an outbreak of the virus in Liberia, Sierra Leone, Guinea and Nigeria is believed to have infected 1,440 people and killed more than 826 this year.
The United States is planning to send 50 health experts to West Africa to help contain the outbreak.
Frieden said the 50 experts from the CDC will work to combat the outbreak and help implement stronger systems to fight the disease.
The Ebola virus causes viral hemorrhagic fever, which affects multiple organ systems in the body and is often accompanied by bleeding.
Early symptoms include sudden onset of fever, weakness, muscle pain, headaches and a sore throat. They later progress to vomiting, diarrhea, impaired kidney and liver function -- and sometimes internal and external bleeding.
The United States had not treated an Ebola patient until last week, but the CDC has spearheaded efforts to prepare for the deadly virus.
It helped create an isolation unit at Emory University Hospital, which is being used to treat American doctor Kent Brantly, who contracted Ebola in Liberia and was evacuated to the facility in Atlanta over the weekend.
A second American patient, Nancy Writebol, is scheduled to arrive from Liberia on Tuesday. She will undergo treatment at the same unit.
Emory is one of four U.S. institutions capable of providing such treatment.
But in the nations hardest-hit and not as prepared, the reality is grim. Even in the best-case scenario, it could take three to six months to stem the epidemic in West Africa, Frieden said.
Ebola spreads through contact with organs and bodily fluids such as blood, saliva, urine and other secretions of infected people. It has no cure. The most common treatment requires supporting organ functions and maintaining bodily fluids such as blood and water long enough for the body to fight off the infection.
Ebola also claimed the life of a medical director at a hospital in Liberia's capital, Monrovia. Dr. Patrick Nshamdze tested positive Tuesday after being sick for two weeks. He died Saturday.
In Sierra Leone, where government officials have asked citizens to stay away from work, the military has deployed at least 750 medical officials to 13 locations, military spokesman Col. Michael Samura said.
Health officials are screening incoming and outgoing passengers at the country's main international airport with a device that takes people's temperature from their eyes at a distance.
Anyone showing signs of fever is quarantined and their blooded in tested.
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CNN's David McKenzie contributed to this report from Freetown, Sierra Leone. Journalist Heather Murdock reported from Nigeria. CNN's Faith Karimi, Nana Karikari-apau and Christabelle Fombu contributed to this report.
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