The outbreak is fast-moving with unprecedented features "delivering one surprise after another," WHO director general Dr Margaret Chan told a European conference of her organisation.
She added: "This is an unforgiving virus that shows no mercy for even the slightest mistake.
"To date, nearly 300 health care workers have been infected and around half of them have died."
She warned of whole countries shutting down - and an absence of hospital beds to take newly infected patients in countries such as Liberia.
“Delivering a baby in Liberia is the most dangerous job on the planet," she said.
She warned the whole world was now at risk and was ill-prepared to face the emergency - as the virus was starting to spread in Nigeria's oil and gas distribution port, Port Harcourt.
She added: "In the simplest terms, this outbreak shows how one of the deadliest pathogens on earth can exploit any weakness in the health infrastructure, be it inadequate numbers of health care staff or the virtual absence of isolation wards and intensive care facilities throughout much of sub-Saharan Africa."
Dr Chan's conclusions were backed yesterday by experts at Warwick University.
They said the outbreak was of an "unprecedented scale" and "out of all proportion."
University mathematicians had thought they had successfully developed a model to predict the impact of outbreaks of the virus.
Researcher Dr Thomas House said: "If we analyse the data from past outbreaks we are able to design a model that works for the recorded cases of the virus spreading and can successfully replicate their eventual size.
"The current outbreak does not fit this previous pattern and, as a result, we are not in a position to provide an accurate prediction of the current outbreak."
* According to WHO estimates, so far some 2,453 people have died in Guinea, Liberia and Sierra Leone.
Almost 70% of students said they had seen patients or colleagues being stereotyped in a survey.
And 63% had experienced "inappropriate humour," according to research Dr Jonathan Broad.
Dr Broad, a year 1 foundation doctor in Devon, reported the findings to the International Conference on Physician Health at BMA House in London.
His survey of 261 students found that just 5% had reported inappropriate behaviour.
He said some students might regard the behaviour as a temporary lapse - but others were worried about the impact on their careers of making a complaint.
He said: “People sometimes do not believe that someone intended to be discriminatory, so do not act. The medical hierarchy is also an important factor; that reporting on someone will detriment the discrimination reporter’s career."
A service providing support to doctors in London has seen a massive increase in young women, according to former Royal College of GPs chair Dr Clare Gerada.
Young female doctors face uncertainty and stress about where they would be living or working - and faced complications in basic living, such as signing tenancies for flats, she said.
Dr Gerada revealed the problem at the International Conference on Physician Health at BMA House in London.
She said that by 2013 the Practitioner Health Programme, which she runs, had helped 101 women in the year compared with 31 men. Four years earlier it had helped 21 women and 27 men.
She said: "The last thing we have at the moment is stability."
His skeleton was excavated underneath a car park in the city of Leicester, UK.
Now a team of pathologists and engineers have reported on their study of his remains in The Lancet.
They say the findings suggest he had lost his helmet at the time of his death during the Battle of Bosworth, which ushered in the reign of the Tudor dynasty.
They found no evidence of injuries to his arms or hands - suggesting they remained well-armoured.
According to William Shakespeare, the king died after losing his horse.
The research also found injuries to his pelvis, that could have been fatal. But the researchers say these were likely inflicted when he was dead or dying - as otherwise his armour would have prevented them.
In total the doomed king suffered 11 injuries, according to the researchers, who used CT scans and micro-CT imaging.
Researcher Professor Guy Rutty, from Leicester University, said: “The most likely injuries to have caused the King’s death are the two to the inferior aspect of the skull—a large sharp force trauma possibly from a sword or staff weapon, such as a halberd or bill, and a penetrating injury from the tip of an edged weapon.
"Richard’s head injuries are consistent with some near-contemporary accounts of the battle, which suggest that Richard abandoned his horse after it became stuck in a mire and was killed while fighting his enemies.”
Perimortem trauma in King Richard III: a skeletal analysis. Lancet 17 September 2014 [abstract]