The tests for the likely disease came back as negative. This was very surprising, new diseases rarely emerge. There are two possibilities:

• A new disease has been discovered.
• There is a problem with the tests and they have failed to pick up a disease which we already know of.

Scientists sometimes use a principle called Occam’s razor to decide out of competing explanations which is the most likely to be correct. Occam’s razor says that the simplest explanation (which adequately explains the situation) is most likely to be correct. Another way of looking at this in medicine is the phrase ‘if you hear hooves think horses, not zebras!’ What this means is that if the somebody has the symptoms of the flu and the same symptoms indicate west nile virus it is much more likely (in most parts of the world) that they have the flu and so the doctor should treat the person for the flu rather than West Nile virus.