LONDON, Sept 23 (Reuters) - West Africa's Ebola epidemic is the largest the world has ever seen, but infectious disease experts are almost as fearful of a long-term legacy in humans as they are about the deaths it is causing right now.

While the current outbreak is vast and out of control, even pessimistic forecasts suggest it will eventually recede.

But if the virus continues to transmit from person to person for a year or more, the risk is that Ebola will become endemic in humans and constitute an ever-present threat to people in the region and the rest of the world.

"The big question here for me is, will this virus become endemic -- meaning it's being transmitted at low levels (in humans all the time)?" said Peter Piot, director of the London School of Hygiene and Tropical Medicine and one of the scientists who identified the Ebola virus almost 40 years ago.

Jeremy Farrar, director of the Wellcome Trust and an expert in infectious diseases, said all the signs are that this Ebola outbreak will run for many months yet, increasing the risk that West Africans could become a reservoir for the virus's spread to other parts of the continent and the rest of the world.

"The concern is that if it keeps going, it will turn from an epidemic disease, which is terrible, to becoming endemic in humans, which means would no longer require an overspill from animals to cause an outbreak," he told reporters at a briefing.

"Then it would also increase the possibility of spread beyond the region."

The West Africa Ebola epidemic has killed more than 2,800 people since it began in Guinea earlier this year, and the World Health Organization (WHO) has said up to 20,000 people could be affected before it is brought under control.

The virus, which can have a human mortality rate of up to 90 percent, is thought to be carried by bats or other wild animals and crosses into humans through contact with blood, meat or other infected fluids.

BAD HOSTS

Farrar said that if Ebola were to become endemic, it would almost inevitably simultaneously become less virulent.

This would mean that while the risk of wide regional and even global spread was far higher, the disease would be likely to kill a smaller proportion of the people it infected.

Ebola infection is caused by a virus whose raison d'etre is to survive for as long as possible so that it can replicate and multiply, Piot explained.

Because so many humans are killed so quickly, they are in fact a very ineffective "host" for the Ebola virus. A mortality rate of up to 90 percent may be frightening, but at least it means the outbreaks eventually kill themselves off.

"We (humans) are a very bad host from the virus' point of view," said Piot. "A host that's killed by a virus in a week or so is absolutely useless.

"So in all other outbreaks it eventually just disappeared from the human host and retreated into animals."

If it were to adapt to humans, perhaps becoming less deadly and allowing them to survive and become better hosts, the virus could settle and pool into a human reservoir.

"The time you really start to worry is when mortality rate drops -- because that suggests the probability that the disease is adapting to humans and risks becoming endemic," said Farrar.

A panel of more than 60 World Health Organization (WHO) experts conducted an analysis of the first nine months of the West Africa outbreak and calculated its case fatality rate at between 69 and 73 percent, according to a study in the New England Journal of Medicine.

But epidemiologists have cautioned that data collection is understandably lagging behind as the disease wreaks havoc in Guinea, Liberia and Sierra Leone - suggesting current case numbers and death rates are likely to be underestimates.

(Reporting by Kate Kelland; Editing by Sonya Hepinstall)