Timeline of the Ebola Virus in America

Ebola, a virus that is affecting people thousands of miles away in West Africa, is now in America with two transmissions on U.S. soil confirmed in Dallas and officials calling additional transmissions to health workers a "very real possibility."

The Ebola outbreak in West Africa has sickened 8,399 people since March, killing 4,033 of them -- making it the worst outbreak o the virus in history, according to the World Health Organization.

Find out how the virus first arrived in the United States -- via U.S. missionaries flown here for treatment over the past summer -- and then how Ebola was unwittingly imported via Thomas Eric Duncan, who flew from Liberia to Texas with the virus and later died in Dallas.

Oct. 16, 2014 - Dallas nurse Nina Pham, 26, the first person to contract Ebola in the United States, is expected to be flown from Texas Health Presbyterian Hospital in Dallas to the National Institutes of Health hospital in Bethesda, Maryland. Pham treated Thomas Eric Duncan, the first person diagnosed with Ebola on American soil, at Texas Health Presbyterian Hospital.

Oct. 15, 2014 - Amber Vinson, 29, another nurse who treated Duncan at Texas Health Presbyterian Hospital, is diagnosed with Ebola shortly after midnight and flown to Emory University Hospital that evening.

Oct. 14, 2014 - Vinson is taken to Texas Health Presbyterian Hospital in Dallas with a fever.

Oct. 13, 2014 - Vinson flies from Cleveland to Dallas on Frontier Airlines Flight 1143, arriving at 8:16 p.m. She has no symptoms, but her temperature was 99.5 degrees that morning. She called the Centers for Disease Control and Prevention before boarding, and no one told her not to fly.

Oct. 12, 2014 - Texas Health Presbyterian Hospital in Dallas says that Pham has tested positive for Ebola.

Oct. 10, 2014 - Vinson takes a commercial flight from Dallas to Cleveland, Ohio, to prepare for her upcoming wedding.

Oct. 9, 2014 - Dallas county Sheriff's deputy who had signs of Ebola tests negative for the virus.

Oct. 8, 2014 - Duncan dies at Texas Health Presbyterian Hospital.

Oct. 6, 2014 - Ashoka Mukpo, 33, a freelance American cameraman who contracted Ebola in West Africa, arrives at Nebraska Medical Center for Ebola treatment.

Oct. 6, 2014 - Dr. Rick Sacra, 51, who contracted Ebola while treating patients in a Liberian maternity ward a month earlier, is released from his second hospitalization since returning to the United States. He had been hospitalized at UMass Memorial Medical Center on Worcester, Massachusetts, with what doctors initially thought was an Ebola relapse but soon diagnosed as a respiratory infection.

Oct. 5, 2014 - Sacra hospitalized in Massachusetts with what doctors fear is an Ebola relapse. They isolate him out of what they said was an abundance of caution.

Oct. 2, 2014 - Mukpo is diagnosed with Ebola in Liberia. He worked for Vice News, NBC News and other outlets.

Sept. 30, 2014 - The CDC confirms that a the first patient -- who would later be identified as Duncan -- has been diagnosed with Ebola on U.S. soil.

Sept. 28, 2014 - Duncan returns to the hospital in an ambulance and is isolated.

Sept. 26, 2014 - Duncan goes to Texas Presbyterian Health Hospital in Dallas with a fever and tells a nurse he has been to Liberia. But he is sent home with antibiotics and Tylenol.

Sept. 25, 2014 - Sacra is deemed virus-free and released from Nebraska Medical Center, where he had spent several weeks in an isolation ward recovering from Ebola.

Sept. 20, 2014 - Duncan arrives in the United States from Liberia to visit family.

Sept. 9, 2014 - An unnamed American Ebola patient arrives at Emory University Hospital for treatment. This patient had been working for the World Health Organization in Sierra Leone. The patient was still there as of Oct. 15, 2014. "Given the national focus on Ebola, particularly with the diagnosis in two health care workers, I want to share the news that I am recovering from this disease, and that I anticipate being discharged very soon, free from the Ebola virus and able to return safely to my family and to my community," the unnamed patient said in a statement released on Oct. 15.

Sept. 5, 2014 - Sacra arrives at Nebraska Medical Center for treatment. He eventually gets a blood transfusion from Brantly.

Sept. 3, 2014 - Sacra was diagnosed with Ebola even though he was treating patients in the maternity ward of the ELWA Hospital in Monrovia, Liberia, not Ebola patients.

Aug. 21, 2014 - Dr. Kent Brantly, 33, is discharged from Emory University Hospital, where he was undergoing treatment for Ebola after contracting it in Africa. In a news conference, he hugs several members of the hospital staff.

Aug. 19, 2014 - Missionary Nancy Writebol, 59, is quietly discharged from Emory University Hospital, where she was undergoing treatment for Ebola. She also contracted the virus doing aid work in Liberia.

Aug. 5, 2014 - Writebol is flown from Liberia to Emory University Hospital in Atlanta, Georgia, for Ebola treatment in its isolation ward.

Aug. 2, 2014 - Brantly is flown from Liberia to Emory for treatment. He surprises everyone by walking out of the ambulance into the hospital in his protective suit.

July 31, 2013 - In Liberia, Brantly gets the first dose of an experimental drug called ZMapp, though it was unnamed at the time. Though doctors initially thought there was only enough for one person, Writebol was administered the drug as well.

July 27, 2014 - Missionary groups report that two Americans are sickened with Ebola while helping patients in Monrovia, Liberia. Brantly and Writebol were working for aid groups Samaritan's Purse and SIM, respectively. Brantly later told reporters he held patients' hands as they were dying.

March 19, 2014 - What would become the largest Ebola outbreak in history begins in March 2014 with 23 deaths from what is then called a "mystery" hemorrhagic fever.

1976 - Ebola is first discovered in what is now the Democratic Republic of Congo near the Ebola River in 1976. Thirty-two Ebola outbreaks would follow, bringing the total number of cases before this outbreak to 2,361, including 1,438 deaths, according to the World Health Organization.