(CNN) Here's a look at Ebola , a virus with a high fatality rate that was first identified in Africa in 1976.

Facts

Ebola hemorrhagic fever is a disease caused by one of five different Ebola viruses. Four of the strains can cause severe illness in humans and animals. The fifth, Reston virus, has caused illness in some animals, but not in humans.

Ebola is extremely infectious but not extremely contagious. It is infectious, because an infinitesimally small amount can cause illness. Laboratory experiments on nonhuman primates suggest that even a single virus may be enough to trigger a fatal infection.

Ebola is considered moderately contagious because the virus is not transmitted through the air.

Humans can be infected by other humans if they come in contact with body fluids from an infected person or contaminated objects from infected persons . Humans can also be exposed to the virus, for example, by butchering infected animals.

Symptoms of Ebola typically include : weakness, fever, aches, diarrhea, vomiting and stomach pain. Additional experiences include rash, red eyes, chest pain, throat soreness, difficulty breathing or swallowing and bleeding (including internal).

Typically, symptoms appear eight to 10 days after exposure to the virus, but the incubation period can span two to 21 days.

Ebola is not transmissible if someone is asymptomatic and usually not after someone has recovered from it. However, the virus has been found in the semen of men who have recovered from Ebola and possibly could be transmitted from contact with that semen.

There are five subspecies of the Ebola virus: Zaire ebolavirus (EBOV), Bundibugyo ebolavirus (BDBV), Sudan ebolavirus (SUDV), Taï Forest ebolavirus (TAFV) and Reston ebolavirus (RESTV).

Click here for the CDC's list of known cases and outbreaks.

2014-2016 West Africa Outbreak

(Full historical timeline at bottom)

March 2014 - The CDC issues its initial announcement on an outbreak in Guinea, and reports of cases in The CDC issues its initial announcement on an outbreak in Guinea, and reports of cases in Liberia and Sierra Leone.

April 16, 2014 - The New England Journal of Medicine publishes a The New England Journal of Medicine publishes a report, speculating that the current outbreak's Patient Zero was a 2-year-old from Guinea. The child died on December 6, 2013, followed by his mother, sister and grandmother over the next month.

August 8, 2014 - Experts at the World Health Organization (WHO) declare the Ebola epidemic ravaging West Africa an international health emergency that requires a coordinated global approach, describing it as the worst outbreak in the four-decade history of tracking the disease.

August 19, 2014 - Liberia's President Ellen Johnson Sirleaf declares a nationwide curfew beginning August 20 and orders two communities to be completely quarantined, with no movement in or out of the areas.

September 6, 2014 - The government of Sierra Leone announces plans for a nationwide lockdown from September 19-21, in order to stop the spread of Ebola. The lockdown is being billed as a predominantly social campaign rather than a medical one, in which volunteers will go door-to-door to talk to people.

September 16, 2014 - calls the efforts to combat the Ebola outbreak centered in West Africa "the largest international response in the history of the CDC." Speaking from the CDC headquarters in Atlanta, Obama adds that "faced with this outbreak, the world is looking to" the United States to lead international efforts to combat the virus. US President Barack Obama calls the efforts to combat the Ebola outbreak centered in West Africa "the largest international response in the history of the CDC." Speaking from the CDC headquarters in Atlanta, Obama adds that "faced with this outbreak, the world is looking to" the United States to lead international efforts to combat the virus.

October 6, 2014 - A nurse's assistant in Spain becomes the first person known to have contracted Ebola outside Africa in the current outbreak. The woman helped treat two Spanish missionaries, both of whom had contracted Ebola in West Africa, one in Liberia and the other in Sierra Leone. Both died after returning to Spain. A nurse's assistant in Spain becomes the first person known to have contracted Ebola outside Africa in the current outbreak. The woman helped treat two Spanish missionaries, both of whom had contracted Ebola in West Africa, one in Liberia and the other in Sierra Leone. Both died after returning to Spain. On October 19, Spain's Special Ebola Committee says that nurse's aide Teresa Romero Ramos is considered free of the Ebola virus.

October 8, 2014 - Thomas Eric Duncan dies of Ebola in Dallas.

October 20, 2014 - Under fire in the wake of Ebola cases involving two Dallas nurses, Under fire in the wake of Ebola cases involving two Dallas nurses, the CDC issues updated Ebola guidelines that stress the importance of more training and supervision, and recommend that no skin be exposed when workers are wearing personal protective equipment, or PPE.

October 23, 2014 - Craig Spencer, Craig Spencer, a 33-year-old doctor who recently returned from Guinea, has tested positive for Ebola -- the first case of the deadly virus in New York and the fourth diagnosed in the United States.

October 24, 2014 - In response to the New York Ebola case, the In response to the New York Ebola case, the governors of New York and New Jersey announce that their states are stepping up airport screening beyond federal requirements for travelers from West Africa . The new protocol mandates a quarantine for any individual, including medical personnel, who has had direct contact with individuals infected with Ebola while in Liberia, Sierra Leone or Guinea. The policy allows the states to determine hospitalization or quarantine for up to 21 days for other travelers from affected countries.

January 18, 2015 - after no new cases in 42 days. Mali is declared Ebola free after no new cases in 42 days.

February 22, 2015 - , and President Sirleaf also lifts a nationwide curfew imposed in August to help combat the virus. Liberia reopens its land border crossings shut down during the Ebola outbreak , and President Sirleaf also lifts a nationwide curfew imposed in August to help combat the virus.

November 2015 - Liberia's health ministry says Liberia's health ministry says three new, confirmed cases of Ebola have emerged in the country.

December 29, 2015 - WHO declares WHO declares Guinea is free of Ebola after 42 days pass since the last person confirmed to have the virus was tested negative for a second time.

January 14, 2016 - A statement is released by the UN stating that "For the first time since this devastating outbreak began, all known chains of transmission of Ebola in West Africa have been stopped and no new cases have been reported since the end of November."

Timeline

*Includes information about Ebola and other outbreaks resulting in more than 100 deaths or special cases.

1976 - First recognition of the EBOV disease is in Zaire (now Democratic Republic of the Congo). The outbreak has 318 reported human cases, leading to 280 deaths. An SUDV outbreak also occurs in Sudan (now South Sudan), which incurs 284 cases and 151 deaths.

1990 - In Texas and Virginia quarantine facilities, four humans develop Ebola antibodies after contact with monkeys imported from the Philippines. None of the humans has symptoms.

1995 - An outbreak in the Democratic Republic of the Congo (DRC) leads to 315 reported cases and at least 250 deaths.

2000-2001 - A Ugandan outbreak (SUDV) results in 425 human cases and 224 deaths.

2001-2002 - An EBOV outbreak occurs on the border of Gabon and Republic of the Congo (ROC), which results in 53 deaths on the Gabon side and at least 43 deaths on the ROC side.

December 2002-April 2003 - An EBOV outbreak in ROC results in 143 reported cases and 128 deaths.

2007 - An EBOV outbreak occurs in the DRC, 187 of the 264 cases reported result in death. In late 2007, an outbreak in Uganda leads to 37 deaths, with 149 cases reported in total.

August 26, 2014-November 2014 - The Ministry of Health in the DRC notifies WHO of an Ebola outbreak in the country. It is the seventh outbreak in the country since 1976, when the virus was first identified near the Ebola river. The outbreak is not related to the ongoing outbreak in Guinea, Liberia, Nigeria and Sierra Leone. A total of 66 cases are reported, which result in 49 deaths.

September 30, 2014 - Dr. Thomas Frieden, director of the CDC, announces Dr. Thomas Frieden, director of the CDC, announces the first diagnosed case of Ebola in the United States. The person has been hospitalized and isolated at Texas Health Presbyterian Hospital in Dallas since September 28.

July 31, 2015 - The CDC announces that a The CDC announces that a newly developed Ebola vaccine is "highly effective" and could help prevent its spread in the current and future outbreaks.

December 19, 2019 - The US Food and Drug administration announces the approval of a vaccine for the prevention of the Ebola virus for the first time in the United States. The vaccine, Ervebo, was developed by Merck and protects against Ebola virus disease caused by Zaire ebolavirus in people 18 and older.

June 1, 2020 - The 11th outbreak of the virus in the DRC is declared.