The charity's Ebola-struck hospital in the capital Monrovia has been shuttered, three of its personnel have now died and another, an elderly Spanish priest with Ebola, has been evacuated to Madrid for treatment.
The latest Ebola victim from the Saint Joseph's Catholic Hospital is Ghanaian priest George Combey, who died of the virus overnight, his Roman Catholic order said in a statement.
"We wish to show our support to George Combey's family at this difficult time," said the Hospitaller Brothers of St. John of God, which created the Juan Ciudad ONGD international aid group.
The haemorrhagic virus killed the hospital's director, Cameroon-born Patrick Nshamdze, on August 2 and a week later it claimed the life of a nun at the same hospital, Chantal Pascaline, from the Democratic Republic of Congo.
It is not known exactly how the third victim contracted Ebola, said the order's spokeswoman, Adriana Castro.
But a test had wrongly shown the first victim, the hospital director, to be free of Ebola before his death, the spokeswoman told AFP.
- 'Did not take precautions' -
"What we fear is that as Patrick's test showed negative, many of them relaxed," she added.
"They probably did not carry on taking such strict safety measures such as not touching and all that because they were treating him. That may be when he was infected."
The evacuated priest, 75-year-old missionary Miguel Pajares who is in isolation at Madrid's Carlos III hospital for treatment with US experimental drug ZMapp, may have caught Ebola in the same way, she said.
"We don't know 100 percent but it is probable that is how Pajares was infected and possibly from Pajares the virus extended among people who were there," the spokeswoman said in an interview.
"Until they knew they were positive with Ebola they did not take precautions," she said.
Ebola causes fever and, in the worst cases, unstoppable bleeding, and can be fatal in 25 to 90 percent of cases, according to the WHO.
The virus spreads by close contact with an infected person through bodily fluids such as sweat, blood and tissue.
The latest outbreak -- which the WHO says is by far the worst since Ebola was discovered four decades ago -- has killed around 55-60 percent of those infected.
- No gloves -
The Spanish religious order declined to comment on letters purportedly from Pajares describing his fear of Ebola and a lack of basic equipment to protect against its spread.
"Today, July 9 we had the first death from Ebola in our hospital. Many of us who work here, including me, were in contact with the deceased even though we did not have gloves to protect ourselves," the missionary was quoted as saying in one of several letters to friends and family quoted in Spanish daily El Mundo at the weekend.
"It is painful but we have to be here. I compare it to war. But it is more dangerous. The enemy at home," said another letter.
The WHO issued advice in April to health care workers treating Ebola patients stressing that it is not always possible to identify infected patients.
When within a metre (three feet) of Ebola patients, health care workers should wear a face shield or a medical mask and goggles, a clean, non-sterile long-sleeved gown, and gloves, it said.