Thursday, October 16, 2014

Ebola hunting

Here's a quick prediction for what happens now that the UK has started screening people for Ebola at Heathrow (soon to be extended to other airports and terminals).  They'll find a lot of people with minor stomach bugs of some sort.

Ebola virus
Image: CDC

There are about 150 new Ebola cases reported each day at the moment, so even if we're generous and assume the true figure is 300, and that symptoms last for 2 weeks, only about 4,000 people in total have it at any given time.  The combined population of the worst affected areas (Guinea, Liberia and Sierra Leone) is about 20 million, so maybe approximately 1 in every 5,000 people are affected.  Fewer than this will be in a state fit to travel (I imagine they would have to be in the early stages of the disease).

Gastroenteritis is endemic pretty much everywhere, and is worse in the developing world because of poor sanitation and sewage treatment.  There are around 3 to 5 billion cases per year, so on the order of half the people in the world get it each year (in reality, young children are more vulnerable than adults).  If it lasts for 4 days on average (it can be much worse, of course), that suggests that on a given day about 1 in 200 people would have it.

From this a fairly generous estimate is that someone screening will see at least 25 people with gastroenteritis for every person who has Ebola.  Since many of the symptoms are pretty similar, I predict that, if anything happens at all, a lot of people will get sent to hospital without much cause.

Note that, in reality, international travellers are much richer than most residents of the west African countries affected; consequently they have better access to sanitation and healthcare when in those countries, and will have a substantially lower risk of infection with Ebola as a result (though this also applies to gastroenteritis).

The moral is that hunting for rare things is a pretty thankless task.

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