A factfile on the deadly Ebola virus, which has broken out in remote northwest Democratic Republic of Congo (DRC) and spread to a city, a development likely to complicate containment efforts.
What it is
Ebola - formally known as Ebola virus disease, or EVD - is severe and often lethal.
The average fatality rate is around 50%, varying from 25% to 90%, according to the World Health Organisation (WHO).
The latest outbreak, publicly declared on May 8, has seen 44 reported cases so far with 23 deaths, according to UN figures. Its epicentre is in the Bikoro area in remote Equateur province. On Thursday the WHO said a case had been recorded in Mbandaka, a city roughly 150km away.
Ebola was first identified in 1976 by a team that included a young Belgian microbiologist, Peter Piot, who later founded UNAIDS, the United Nations' spearhead agency against HIV/AIDS.
They named the virus after a river in the Democratic Republic of Congo - then known as Zaire - that was close to the location of the first known outbreak.
Four of the virus species are known to cause disease in humans - Zaire, Sudan, Bundibugyo and Tai Forest.
The virus' natural reservoir animal is probably the bat, which does not itself fall ill, but can pass the microbe on to humans who hunt it for food.
Chimpanzees, gorillas, monkeys, forest antelope and porcupines can also become infected with Ebola, which makes them potential vectors for transmission if they are killed and eaten.
Among humans, the commonest form of infection is through close contact with the blood, body fluids, secretions or organs of someone who is sick with Ebola or has recently died - a risk in African cultures where relatives typically touch the body of the deceased at funerals.
The WHO says it is unclear whether the virus may be transmitted through sexual intercourse, but urges safe-sex practices among all Ebola survivors and their sexual partners.
Those infected do not become contagious until symptoms appear - something that happens after an incubation period of between two and 21 days.
High fever, weakness, intense muscle and joint pain, headaches and a sore throat are often followed by vomiting and diarrhoea, skin eruptions, kidney and liver failure, internal and external bleeding.
After-effects have often been observed in survivors, including arthritis, problems with vision, eye inflammation and hearing difficulties.
At present, there is no licenced drug to prevent or treat Ebola. However, a range of experimental drugs are in development and a batch of prototype vaccine has been rushed to the DRC. Early care with rehydration may boost the chance of survival.
In the absence of a tried-and-tested pharmaceutical weapon against Ebola, health experts have responded with time-honoured measures of control, prevention and containment.
They use rigorous protocols to protect medical personnel with disposable full-body suits, masks, goggles, gloves and disinfecting sprays.
Controlling the spread in the community is combatted by tracing and isolating people who have been in contact with an Ebola victim. Enlisting the support of the community through awareness campaigns is vital.
The world's worst Ebola outbreak started in December 2013 in southern Guinea before spreading to neighbouring west African countries Liberia and Sierra Leone.
That outbreak killed more than 11 300 people out of nearly 29 000 registered cases, according to WHO estimates.
The real figure may have been significantly higher.
Read more about Ebola here
More than 99% of victims were in Liberia, Guinea and Sierra Leone although cases popped up all over the world, sparking panic.
The WHO declared the epidemic over in January 2016, although this was followed by flare-ups in all three countries.
Before the West African outbreak, Ebola killed about 1 700 people over four decades. The DRC has had nine known outbreaks.
Rolling back Ebola is a major challenge for any country, especially in urban environments where people are more mobile and come into more contact with others than in rural areas.
But experts say this task is greatly magnified in the DRC, one of the world's most vulnerable and volatile countries.
A country four times the size of France, the vast nation has been unstable and episodically racked by violence since independence from Belgium in 1960.
Despite vast mineral wealth, it remains mired in poverty, and is saddled with a reputation for corruption and poor governance. Basic infrastructure - hospitals, roads, electricity - is a major obstacle in remote areas.
SOURCES: WHO, US Centers for Disease Control and Prevention (CDC), European Centre for Disease Prevention and Control (ECDC)