The ebola virus that has killed almost 1,000 people in West Africa this year is fatal for "up to 90%" of those infected, according to the World Health Organization (WHO). But note the words "up to"... What is the normal fatality rate, asks James Fletcher?
The WHO describes ebola as "one of the world's most virulent diseases". It is, according to the organisation's website, "a severe, often fatal illness, with a case fatality rate of up to 90%".
A case fatality rate - or CFR - is a relatively simple measurement. It's the number of people who die from an illness divided by the number of people diagnosed with it. But in the current outbreak, the proportion of infected people dying is far lower than 90%.
"That 90% figure actually comes from one outbreak of ebola which took place in the Congo between 2002 and 2003. It's the highest rate we have ever seen," says Maimuna Majumder, a biostatistician and epidemiologist at the Massachusetts Institute of Technology.