AS HAITI'S deadly cholera epidemic spreads, it may seem irrelevant to ask where the disease came from. The World Health Organization certainly thinks it is, describing the question as "unimportant".
That could not be further from the truth. Haitians themselves care deeply about how their country got cholera. There is widespread suspicion that the disease was brought in by United Nations peacekeepers from Nepal, and that the UN is now covering it up. This suspicion has sparked riots that have killed people, both directly and by impeding medical efforts.
We should care too. Haiti's cholera tragedy - more than 1600 dead and 30,000 hospitalised as New Scientist went to press - tells us something important about our highly interconnected planet, and how we should - but still don't - govern it.
Cholera bacteria thrive on poverty and disruption, and Haiti has plenty of both. The country was free of cholera when the earthquake struck in January, but when the disease broke out in October it quickly took off.
When the news broke on 20 October, suspicion fell rapidly on 454 Nepalese UN peacekeepers based in the town of Mirebalais, 60 kilometres north of the capital Port-au-Prince. Haitian officials tested the river by the base two days later.
There were reasons to suspect these Nepalese. Cholera, which is carried by faeces-tainted water, is endemic in Nepal: there was an outbreak in Kathmandu, the country's capital, just before the peacekeepers flew in from there between 9 and 16 October. Their camp in Mirebalais dumped sewage straight into a stream that led to Haiti's main central river. The first cases were in Mirebalais and downstream, areas barely touched by the earthquake. What is more, the DNA in Haiti's cholera shows it was a single, recent introduction of a strain from south Asia, though we don't know if it is circulating in Nepal.
All of this is just circumstantial evidence, of course. The UN insists it is in the clear because the tests on water on or near the base did not find cholera, and none of the peacekeepers had symptoms.
Yet this doesn't clear the matter up. Many people with the strain now circulating in Haiti do not develop symptoms but shed bacteria in their faeces up to two weeks after infection. Nor are negative water tests conclusive: cholera researchers say the bacteria are hard to find in fast-flowing rivers. To settle the matter, the Nepalese soldiers themselves should have been tested, promptly.
A single positive swab from a soldier early in the outbreak would have strongly suggested they were the source. A negative result would not have entirely cleared them - tests can produce false negatives - but it may well have calmed public suspicion.
But no such tests were done. The Nepalese government claims the water samples alone prove that its troops are not the source. The UN Mission in Haiti even phoned me out of the blue to claim that tests cannot detect cholera in symptom-free people.
They can. That is an elementary scientific fact about cholera.
Why would the UN go to such trouble? I can only conclude that they are trying to protect themselves and their people. Many Haitians dislike the UN force; dozens of peacekeepers have been killed in violent clashes since the mission arrived in 2004 to stabilise the country in the face of political upheaval.