International agencies and governments are fighting to contain the world's worst Ebola epidemic since the disease was identified in 1976.
The virus, which causes fever and bleeding, has killed at least 3,338 people.
Here is a timeline of the outbreak:
March 22: Guinea confirms a previously unidentified hemorrhagic fever, which killed over 50 people in its southeastern Forest Region, is Ebola. Cases are also reported in the capital.
March 30: Liberia reports two Ebola cases; suspected cases reported in Sierra Leone.
April 1: Medical charity Medecins Sans Frontieres (MSF) warns the epidemic's spread is "unprecedented." But a World Health Organization (WHO) spokesman calls it "relatively small still."
April 4: A mob attacks an Ebola treatment center in southeastern Guinea. Healthcare workers in Guinea, Sierra Leone and Liberia face growing hostility from fearful, suspicious local people.
May 26: WHO confirms first Ebola deaths in Sierra Leone.
June 17: Liberia says Ebola has reached its capital, Monrovia.
June 23: With deaths above 350, making the West African outbreak the worst Ebola epidemic on record, MSF says it is "out of control" and calls for massive resources.
July 25: Nigeria, Africa's biggest economy, confirms its first Ebola case, a man who died in Lagos after traveling from Monrovia.
July 29: Dr. Sheik Umar Khan, who was leading Sierra Leone's fight against the epidemic, dies of Ebola.
July 30: Liberia shuts schools and quarantines the worst-affected communities, using troops for enforcement.
Aug. 2: A U.S. missionary physician infected with Ebola in Liberia is flown to Atlanta in the United States for treatment.
Aug. 5: A second U.S. missionary infected with Ebola is flown from Liberia to Atlanta for treatment.
Aug. 8: WHO declares Ebola "international public health emergency," stops short of urging ban on trade and travel.
Aug. 12: WHO says death toll has topped 1,000, approves use of unproven drugs or vaccines.
A Spanish priest with Ebola dies in a Madrid hospital.
Aug. 15: MSF says the epidemic will take about six months to control.
Aug. 20: Security forces in Monrovia fire shots, tear gas to disperse crowd trying to break out of quarantine, killing a teenager.
Aug. 21: The two U.S. missionary aid workers treated in Atlanta are released from the hospital free of the virus.
Aug. 24: Democratic Republic of Congo declares Ebola outbreak in a northern province, apparently separate from larger outbreak.
An infected British medical worker is flown home from Sierra Leone for treatment.
Aug. 28: WHO puts death toll at above 1,550, warns outbreak could infect more than 20,000.
Aug. 29: Senegal reports first confirmed Ebola case.
Sept 2: MSF President Joanne Liu tells U.N. members the world is "losing the battle" to contain Ebola, slams "a global coalition of inaction."
Sept. 3: Epidemic's pace accelerates; deaths top 1,900. Officials say there were close to 400 deaths in the past week.
A third U.S. missionary doctor infected with Ebola is flown out of Liberia for treatment in Omaha, Nebraska.
Sept. 5: Latest WHO tally: More than 2,100 dead out of about 4,000 people thought to have been infected.
Sept. 7: President Barack Obama says in an interview the United States needs to do more to help control Ebola to prevent it from becoming a global crisis.
Sept. 8: Britain says it will send military and humanitarian experts to Sierra Leone to set up a treatment center; United States says it will send 25-bed military field hospital to Liberia to care for health workers.
A fourth Ebola patient will be flown to the United States for treatment in Atlanta.
Sept. 9: New WHO tally: At least 2,296 dead out of 4,293 cases recorded in five countries.
Sept. 13: Liberian President Ellen Johnson Sirleaf appeals to Obama for urgent aid in tackling Ebola.
Sept. 16: The United States promises to send 3,000 military engineers and medical personnel to West Africa to build clinics and train healthcare workers.
New WHO tally: 2,461 dead out of 4,985 infected, a doubling of the death toll in the past month.
Sept. 17: MSF says a French nurse volunteering for the medical charity in Liberia has Ebola.
Sept. 18: New WHO tally: 2,630 dead out of 5,357 believed infected.
The United Nations says a special mission to combat Ebola will deploy staff in Liberia, Guinea and Sierra Leone. U.N. Security Council adopts U.S.-drafted resolution calling for the lifting of travel and border restrictions.
French President Francois Hollande says a military hospital will be set up in Guinea.
Sept. 19 - Streets in Sierra Leone's capital, Freetown, are deserted as the country imposes a controversial three-day lockdown to try to halt Ebola's spread.
Sept. 20 - Thomas Eric Duncan flies from Liberia to Dallas via Brussels and Washington after reportedly trying to help a woman with Ebola in his home country.
Sept. 22 - WHO says the outbreak has been largely contained in Senegal and Nigeria but says Ebola has killed more than 2,811 people in West Africa.
Sept. 23 - The CDC estimates between 550,000 and 1.4 million people in West Africa may be infected with Ebola by January.
Sept. 25 - At a meeting on the sidelines of the U.N. General Assembly, Obama calls on more countries to help fight Ebola, saying hundreds of thousands of lives are at stake.
Duncan goes to a Dallas hospital complaining of fever and abdominal pain. He is sent back to the apartment where he is staying, with antibiotics, despite telling a nurse he has traveled from West Africa.
Sept. 26 - New WHO tally: 3,091 dead out of 6,574 probable, suspected and confirmed cases.
Cuba says it will send nearly 300 doctors and nurses to West Africa, to join 165 healthcare workers slated to arrive in early October.
Sept. 28 - Duncan's condition worsens and is taken to hospital by ambulance.
Sept. 30 - CDC confirms Duncan has Ebola, making his first case diagnosed in the United States.
Oct. 1 - WHO updates West Africa death toll to 3,338 dead out of 7,178 cases.
Wall Street stocks fall partly on Ebola concerns, with airline and hotel shares taking hits.
Oct. 2 - Health officials say Duncan may have come into contact with up to 100 people.
Britain pleads for international help to fight epidemic at "Defeating Ebola" conference in London.