Global health authorities are struggling to contain the world's worst Ebola epidemic since the disease was identified in 1976. The virus has killed more than 4,500 people.

Here is a timeline of the outbreak:

March 22: Guinea confirms a previously unidentified hemorrhagic fever, which killed more than 50 people, is Ebola.

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." A World Health Organization (WHO) spokesman calls it "relatively small still."

April 4: Mob attacks Ebola treatment center in Guinea. Healthcare workers there and in Sierra Leone and Liberia face hostility from fearful, suspicious people.

May 26: WHO confirms first Ebola deaths in Sierra Leone.

June 17: Liberia reports Ebola in its capital, Monrovia.

June 23: With deaths above 350, making the West African outbreak the worst on record, MSF says it is "out of control" and calls for massive resources.

July 25: Nigeria 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, 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."

Aug. 12: WHO says death toll tops 1,000, approves use of unproven drugs or vaccines.

A Spanish priest with Ebola dies in Madrid hospital.

Aug. 15: MSF says 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 teen.

Aug. 21: Two U.S. missionary aid workers treated in Atlanta are released from hospital August 19 and 21 free of the virus.

Aug. 24: Democratic Republic of Congo declares Ebola outbreak, believed separate from West Africa epidemic.

Infected British medical worker is flown home from Sierra Leone for treatment.

Aug. 28: WHO puts death toll above 1,550, warns outbreak could infect more than 20,000.

Aug. 29: Senegal reports first confirmed Ebola case.

Sept 2: MSF president tells United Nations the world is losing battle to contain Ebola, slams "global inaction".

Sept. 3: Epidemic accelerates; deaths top 1,900. Officials say close to 400 deaths in past week.

Third U.S. 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: President Barack Obama says United States needs to do more to help prevent Ebola from becoming a global crisis.

Sept. 8: Britain to send military and humanitarian experts to Sierra Leone to set up treatment center; United States to send field hospital to Liberia to care for health workers.

Fourth Ebola patient will be flown to United States for treatment in Atlanta.

Sept. 9: WHO says at least 2,296 dead out of 4,293 cases recorded in five countries.

Sept. 13: Liberia appeals to Obama for aid to fight Ebola.

Sept. 16: United States promises to send 3,000 military engineers and medical personnel to West Africa to build clinics and train healthcare workers.

WHO says 2,461 dead out of 4,985 infected, doubling death toll in the past month.

Sept. 17: MSF says French nurse volunteer in Liberia has Ebola.

Sept. 18: WHO says 2,630 dead out of 5,357 thought infected.

United Nations special mission to combat Ebola will deploy staff in Liberia, Guinea, Sierra Leone. Security Council adopts resolution calling for lifting travel, border restrictions.

French President Francois Hollande says military hospital will be set up in Guinea.

Sept. 19 - Streets in Sierra Leone's capital, Freetown, are deserted under three-day lockdown to try to halt Ebola's spread.

Sept. 20 - Liberian Thomas Eric Duncan flies from Liberia to Dallas via Brussels and Washington after trying to help woman with Ebola in his home country.

Sept. 22 - WHO declares outbreak largely contained in Senegal and Nigeria but says Ebola has killed more than 2,811 people in West Africa.

Sept. 23 - U.S. Centers for Disease Control and Prevention (CDC) estimates between 550,000 and 1.4 million people in West Africa may have Ebola by January.

Sept. 25 -Duncan goes to Dallas hospital with fever, abdominal pain. He is sent back to apartment where he is staying despite telling a nurse he traveled from West Africa.

Sept. 26 - New WHO tally: 3,091 dead out of 6,574 probable, suspected and confirmed cases.

Cuba says will send nearly 300 doctors and nurses to West Africa to join 165 healthcare workers due there early October.

Sept. 28 - Duncan returns by ambulance to Dallas hospital.

Sept. 30 - CDC confirms Duncan has Ebola; first case diagnosed in the United States.

Oct. 1 - WHO says 3,338 dead out of 7,178 cases in West Africa.

Oct. 2 - Britain pleads for international help to fight epidemic at conference in London.

NBC News says American freelance cameraman in its employ, Ashoka Mukpo, has Ebola; will be flown to United States for treatment.

Oct. 3 - WHO says 3,439 dead out of 7,492 suspected, probable and confirmed cases in West Africa and the United States, which has one.

Ugandan doctor with Ebola arrives in Frankfurt from Sierra Leone for treatment.

Oct. 4 - Volunteer nurse in Liberia who was first French national to contract Ebola leaves hospital outside Paris after being successfully treated for the disease.

Oct. 6 - Spanish nurse is infected with Ebola; she treated infected Spanish priest who was repatriated to Madrid and died.

Cameraman Mukpo in Omaha; taken to Nebraska Medical Center.

Oct. 8 - Duncan, the first person diagnosed with Ebola in the United States, dies in Dallas hospital.

U.S. government orders five major airports to screen passengers from West Africa for fever.

Oct. 9 - WHO revises Ebola death toll to 3,865 out of 8,033 cases, says there is no evidence epidemic is being brought under control in West Africa.

Britain to screen passengers entering country through London's two main airports and Eurostar rail link with Europe.

Some lawmakers call for United States to ban travelers from the West African countries hit hardest by Ebola.

Oct. 10 - WHO raises death toll to 4,033 out of 8,399 cases in seven countries. Most fatalities are in Liberia, Sierra Leone and Guinea.

Oct. 11 - Medical teams at New York's John F. Kennedy International airport begin screening travelers from three West African countries for Ebola symptoms.

Oct. 12 - Nurse in Dallas tests positive for Ebola, becoming first person to contract the virus in the United States. Nina Pham was infected while caring for Duncan at Texas Health Presbyterian Hospital.

Oct. 14- At Heathrow, London's busiest airport, Britain begins screening travelers from West Africa.

Sudanese U.N. medical official who contracted Ebola in Liberia dies in German hospital.

Oct. 15 - Officials say second Texas nurse who treated Duncan has contracted Ebola. Amber Vinson will be treated at Emory University Hospital in Atlanta.

Authorities say Vinson took flight from Cleveland to Dallas/Fort Worth International Airport while running slight fever.

WHO raises death toll to 4,493 people out of 8,997 cases; says epidemic still spreading in West Africa.

Oct. 16 - U.S. congressional subcommittee sharply questions health officials about response to Ebola in United States.

U.S. National Institutes of Health says nurse Pham will be moved from Dallas to an NIH isolation unit in Bethesda, Maryland.

Oct. 17 - WHO raises death toll to 4,546 people out of 9,191 cases.

Senegal declared free of Ebola.

U.S. President Barack Obama appoints Ebola "czar."

Oct. 19 - Spanish nurse appears to be free of Ebola.

Oct. 20 - Nigeria declared free of Ebola.

Unidentified American who contracted Ebola in Sierra Leone is released from Emory University Hospital in Atlanta.

In Texas, 43 people taken off Ebola watch lists. About 260 still being monitored in Texas and Ohio.

United States issues stricter guidelines for health workers treating Ebola victims. No skin or hair exposed.