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 nearly 4,000 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 Médecins Sans Frontières (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 centre 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 travelling 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 US missionary physician infected with Ebola in Liberia is flown to Atlanta in the United States for treatment.
Aug 5: A second US missionary infected with Ebola is flown from Liberia to Atlanta for treatment.
Aug 8: WHO declares Ebola an "international public health emergency".
Aug 12: WHO says the death toll has topped 1,000 and approves the 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 and tear gas is used to disperse crowds trying to break out of quarantine, killing a teenager.
Aug 21: The two US missionary aid workers treated in Atlanta are released from the hospital free of the virus.
Aug 24: Democratic Republic of Congo declares Ebola outbreak, apparently separate from the larger epidemic. An infected British medical worker is flown home from Sierra Leone for treatment.
Aug 28: WHO puts the death toll at above 1,550 and warns that the outbreak could infect more than 20,000.
Aug 29: Senegal reports its first confirmed Ebola case.
Sept 2: MSF President Joanne Liu tells UN members the world is "losing the battle" to contain Ebola and slams "a global coalition of inaction".
Sept 3: The epidemic's pace accelerates and deaths top 1,900. Officials say there were close to 400 deaths in the past week. A third US missionary doctor infected with Ebola is flown from 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: US 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 centre. United States says it will send a 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 Mr 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. The UN Security Council adopts a US-drafted resolution calling for 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: Liberian national 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 UN General Assembly, Mr 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 travelled 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 he is taken to the Dallas hospital by ambulance.
Sept 30: CDC confirms Duncan has Ebola, making his the first case diagnosed in the United States.
Oct 1: WHO updates west Africa death toll to 3,338 dead out of 7,178 cases.
Oct 2: Britain pleads for international help to fight epidemic at "Defeating Ebola" conference in London. NBC News says an American freelance television cameraman working for the network, Ashoka Mukpo, has tested positive for Ebola and will be flown back to the United States for treatment.
Oct 3: New WHO tally: 3,439 dead out of 7,492 suspected, probable and confirmed cases in west Africa and in the United States, which has one of the cases. A Ugandan doctor suffering from the disease arrives in Frankfurt from Sierra Leone for treatment at a hospital there.
Oct 4: The volunteer nurse in Liberia, who was the first French national to contract the virus, leaves a hospital outside Paris after being successfully treated for the disease. Duncan's condition worsens from serious to critical in the US.
Oct 6: A Spanish nurse who treated the Spanish priest who was repatriated to Madrid with Ebola has also been infected, according to health officials in Spain. Freelance TV cameraman Mukpo, 33, lands in Omaha in a private plane and is taken to the Nebraska Medical Centre.
Oct 8: Duncan dies in Dallas hospital. WHO raises Ebola toll to 3,879 dead out of 8,033 cases and says there is no evidence the epidemic is being brought under control in west Africa. The US government orders five major airports to screen passengers from west Africa for fever, in an effort to bolster defences against the spread of Ebola.