WHO says more than 120 health care workers - including senior doctors - have died in the outbreak in west Africa.
And it has warned that international agencies are now struggling to recruit medical staff to work on the outbreak because of the high risks.
The problems are escalating amid reports that the outbreak has now reached the Democratic Republic of Congo.
WHO has now pulled its staff out of Kailahun, Sierra Leone, after one of them fell ill with the virus.
However an investigatory team has been sent to the town to find out how the worker became infected.
Dr Daniel Kertesz, from WHO, said: “This was the responsible thing to do. The field team has been through a traumatic time through this incident.
“They are exhausted from many weeks of heroic work, helping patients infected with Ebola. When you add a stressor like this, the risk of accidents increases.”
WHO said healthcare workers had proved vulnerable because the outbreak had spread beyond the areas where Ebola is endemic to large cities - requiring inexperienced staff to care for patients.
There have also been difficulties distinguishing infected patients with those suffering from diseases such as malaria, typhoid fever and Lassa fever.
It said in some cases workers did not have access to protective equipment whilst when they did have it, it was "hot and cumbersome." Long shifts could also lead to mistakes.
A spokesman said: "The heavy toll on health care workers in this outbreak has a number of consequences that further impede control efforts. It depletes one of the most vital assets during the control of any outbreak.
"WHO estimates that, in the three hardest-hit countries, only one to two doctors are available to treat 100,000 people, and these doctors are heavily concentrated in urban areas.
"It can lead to the closing of health facilities, especially when staff refuse to come to work, fearing for their lives. When hospitals close, other common and urgent medical needs, such as safe childbirth and treatment for malaria, are neglected.
"The fact that so many medical staff have developed the disease increases the level of anxiety: if doctors and nurses are getting infected, what chance does the general public have? In some areas, hospitals are regarded as incubators of infection and are shunned by patients with any kind of ailment, again reducing access to general health care.
"The loss of so many doctors and nurses has made it difficult for WHO to secure support from sufficient numbers of foreign medical staff."
Medicins Sans Frontieres, which revealed the outbreak in the Congo, said it was not yet established it was linked to the west African outbreak.
* Nurse William Pooley is now being treated with the monoclonal antibody drug ZMapp at the Royal Free Hospital in London, it was reported.
WHO officials want e-cigarettes banned in public places and work places and an end to promotion of the products to non-smokers and young people.
WHO's intervention comes amid controversy about the benefits of e-cigarettes in helping to cut the risks from smoking.
Guidance in the UK says people who say they are using them to help give up cigarettes should not be discouraged - but there is not yet evidence that they help quitting.
The proposals are due to be discussed at a summit on tobacco control in Moscow, Russia, in October.
E-cigarettes were invented in China in 2005 and are now worth three billion dollars a year globally - with 466 brands on the market and 8,000 different flavours.
WHO warns that experimentation among teenagers is growing rapidly.
And it warns that the steam from e-cigarettes is not simply water vapour but may contain nicotine and toxic substances.
It is calling for governments to ban fruit, candy and alcohol flavours until it can be proved they are not attractive to children and adolescents.
Italian researchers found that the drug telmisartan can counteract the rise in blood pressure up to a height of 3,400 metres but not beyond.
Kathmandu in Nepal is 1,355 metres above sea level while Denver in the USA is 1,560 metres above sea level.
The researchers camped at an Everest base camp some 5,400 metres above sea level. The mountain peak is 8,800 metres high.
They say theirs is the first study of its kind because they used ambulatory blood pressure monitoring to track changes to blood pressure throughout periods of 24 hours.
The researchers say their findings have implications not just for high altitudes but for others who may suffer from oxygen deprivation.
They found that at the level of the base camp, volunteers had experienced an increase in systolic blood pressure of 14 points and in diastolic blood pressure of ten points.
The findings have been published in the European Heart Journal.
Researcher Professor Gianfranco Parati, of the Istituto Auxologico Italiano, Milan, Italy, said: “Our study provides the first systematic demonstration that exposure to progressively higher altitudes is associated with a progressive and marked increase in ambulatory blood pressure.
"The increase occurred immediately after the high altitude was reached, persisted during prolonged altitude exposure, was seen throughout the 24-hour period but was particularly pronounced at night when there was a reduction in the night-time ‘dip’, and disappeared after return to sea level.
"After reaching Everest base camp, the effect of high altitude was greater on systolic blood pressure in people aged 50 and over compared with younger people."
He added: "Our findings will also enable us to take appropriate action to warn cardiovascular patients of the need for caution whenever they are going to be exposed to high altitudes for leisure or work."
Gianfranco Parati et al. Changes in 24 h ambulatory blood pressure and effects of angiotensin II receptor blockade during acute and prolonged high-altitude exposure: a randomized clinical trial. European Heart Journal 27 August 2014; doi:10.1093/eurheartj/ehu275
The researchers say that social inequalities in salt consumption have hardly changed since the beginning of the century.
The study found an average reduction in salt use of 0.9g a day.
The findings, reported in BMJ Open, involved an analysis of the British National Diet and Nutrition Survey, which involves more than 1,000 people.
Researcher Professor Francesco Cappuccio said: “Whilst we are pleased to record an average national reduction in salt consumption coming from food of nearly a gram per day, we are disappointed to find out that the benefits of such a programme have not reached those most in need.
“The diet of disadvantaged socio-economic groups tends to be made up of low-quality, salt-dense, high-fat, high-calorie unhealthy cheap foods. We have seen a reduction in salt intake in Britain thanks to a policy, which included awareness campaigns, food reformulation and monitoring.
"However, clearly poorer households still have less healthy shopping baskets and the broad reformulation of foods high in salt has not reached them as much as we would have hoped.”
BMJ Open 27 August 2014