The Philippine government and WHO Friday refused to rule out the possibility that five men may have contracted a non-lethal strain of the Ebola virus from infected pigs.
Four pig farm workers and a butcher in the northern Philippines tested positive for antibodies for Ebola-Reston, indicating that they may have been infected with the virus in the past, Health Secretary Francisco Duque said.
Pigs in the same area of the country were struck by the virus over the past year.
If a link is proved between the outbreak in the animals and the men, it would be the first time humans have contracted the disease from pigs, although there was no firm evidence of that so far, officials said.
A World Health Organization-led UN team said all the men - who are aged between 22 and 52 - are well and no longer carry the virus after being able to expel it from their systems.
Julie Hall, a member of the UN investigating team, told a news conference that it was a "low-risk situation and an important situation for animal and human health."
However, she said she agreed with an earlier statement by Duque that a swine-to-human transmission of the virus "cannot be dismissed".
"There are still a lot of unanswered questions about this particular virus," Hall said.
"These are humans who are young, fit and healthy," she said, adding there was no telling what the virus could have done "if other individuals who do not have robust immune systems were to be infected."
The UN sent a team of medical specialists to the Philippines last month after four pigs tested positive for a strain of Ebola that was first found in monkeys exported by the Philippines to a US laboratory in Reston, Virginia.
The other known Ebola strains are deadly to humans.
Two farms in Bulacan and Pangasinan provinces were quarantined and the health department and the UN team gathered blood samples from 77 people who may have been exposed to the virus.
Duque said three of the five men were from the quarantined farms, but all the men are "healthy and have not been seriously ill in the previous 12 months".
The workers are not the first human cases of the Ebola-Reston virus.
Twenty-five people who came into contact with the infected laboratory monkeys in 1989 tested positive for the virus. Only one showed signs of sickness, suffering from flu-like symptoms, but quickly recovered.