What you need to know about the world's worst-ever Ebola outbreak

The world's worst known outbreak of Ebola is officially over, health officials said on Thursday, as Liberia was declared to free of the deadly haemorrhagic virus that started in 2013 and swept through three West African nations.

The declaration comes because it is 42 days since Liberia's last Ebola patient tested negative. The country previously declared itself virus-free in May and September last year but each time a fresh cluster of cases appeared.

Here are some key facts and figures about the epidemic, which killed more than 11,300 people out of 28,600 cases, almost all in the three worst-affected nations of Liberia, Guinea and Sierra Leone:

* The epidemic began in eastern Guinea in December 2013 and swept through Liberia and Sierra Leone. Liberia was the last nation to be declared Ebola-free

* The Ebola virus infected more than 28,600 people and killed 11,300 of them in the three worst affected nations - more cases and more deaths than in all previous outbreaks combined

* Ebola cases were also recorded in seven other countries, including the United States, Spain and Nigeria, but on a much smaller scale, totalling 36 cases and 15 deaths

* The World Health Organization (WHO) declared the West Africa Ebola outbreak a Public Health Emergency of International Concern on Aug. 8, 2014

* After a slow initial response, the WHO and nations ranging from Cuba to France poured in trained staff, field hospitals, laboratories and equipment to tackle the epidemic

* On average, around 50 percent of humans infected in an outbreak die, though in past outbreaks the fatality rate has varied from 25 to 90 percent

* The virus is transmitted to people from wild animals such as fruit bats and monkeys and spreads among humans through contact with bodily fluids of an infected person

* Medical experts believe new Ebola cases in Liberia after it had twice been declared Ebola-free were due to sexual contact as the virus lives in semen longer than the 21-day incubation period elsewhere in the body

* The worst affected countries were, at the start of the epidemic, recovering from years of conflict and instability and had weak health systems and a shortage of expertise and infrastructure

* The Ebola virus first appeared in 1976 in two simultaneous outbreaks, one in Nzara, South Sudan, the other in Yambuku, Democratic Republic of Congo, in a village near the River Ebola

Here are key developments in the three most severely affected countries:

Liberia

* Recorded more than 10,700 cases and 4,800 deaths

* Declared Ebola-free in May 2015 and September 2015, but each time a fresh cluster of cases appeared later.

* Declared Ebola-free on Jan. 14, 2016

Sierra Leone

* First case confirmed in May 2014

* Recorded about 14,100 cases and 4,000 deaths

* Declared Ebola-free on Nov. 7, 2015

Guinea

* The region's outbreak began in Gueckedou, eastern Guinea in December 2013

* Recorded more than 3,800 cases and 2,500 deaths

* Declared Ebola-free on Dec. 29, 2015