Five experts from Switzerland and the United States have spent the last two weeks at the source of the outbreak in a northern suburb of Luanda, gathering blood samples from health clinics where some of the victims had been treated.
"There are various hypotheses we're drawing on. Give us time to work," one WHO source told AFP on condition of anonymity.
The UN agency said it was important that "people do not panic" and report to hospital as soon as they display symptoms of the disease.
All the victims have complained of dizziness, sleepiness, and difficulty in walking and speaking.
The WHO has so far ruled out the possibility that the illness is a virus as it does not transmit from one person to another.
However another of the WHO experts admitted to being at a loss over the disease, being quoted on a local radio station as saying: "I've never seen something like this."
The WHO team is made up of an expert in clinical toxicology, epidemiologists, and environmental health experts who are all working in tandem with the Angolan ministry of health.
First reports of the mystery illness were recorded at the beginning of last month. Although there have been no new deaths since the start of the month, the number of those infected is still growing at a rate of around 20 people a day.
The south-western Africa country's public health infrastructure was ravaged by 27-year civil war which finally ended in 2002.
With drinking water and proper sanitation still a luxury, Angola's citizens have been particularly vulnerable to diseases such as cholera, which killed more than 400 people in the first six months of the year.