Number of UK cases
How many people in the UK have swine flu? Do you want the official or the unofficial figure?
The number of laboratory-confirmed cases stood last night at 404. But there are undoubtedly some cases which have gone undetected. How many?
, a virologist at Queen Mary School of Medicine, University of London, has come up with a figure. Speaking this morning on 5 Live Breakfast, he speculated that there could be 20,000 infected people in the UK.
His theory goes that most of them have had such mild illness, it's gone undetected. I spoke to him afterwards and he stressed that this is a pure "guesstimate" rather than a scientific analysis of the figures. He stressed that the number was not meant to be alarming - and that quite the contrary, it shows that the virus causes mild symptoms in the vast majority of cases. 
, Director of Public Health Strategy at the Health Protection Agency agreed that there are some undetected cases out there, but he didn't think that it was tens of thousands. He said that the UK had among the best flu surveillance systems in the world and doubted there could be that many cases.
That may leave you none the wiser. We may get a better idea by looking at what's happening in the United States.
There, the says that there have now been 11,054 confirmed and probable cases. But it has also repeatedly stated that possibly 19 out of 20 cases are going undetected. As I said in a post on Tuesday, this could mean that there are 200,000 or more people in the United States who have been infected.
So can you simply multiply the current official UK figure of 404 by 20, giving us around 8,000 cases here? It's quite a lot less than Professor Oxford's guesstimate, though still a lot.
But there are two differences I can think of between the UK and the US which may make that figure lower.
First, the virus is not as well established here as it is in North America. We are seeing cases pop up out of the blue, but most are linked to current outbreaks.
Second, the UK does have a very well organised flu surveillance service, one of the features of our GP network set within a national health service. Contrast that with the fragmented healthcare on offer in the US and it's little wonder that surveillance here is likely to be better.
All that probably means that fewer cases are going undetected here than in the US. Whether it's a few hundred or many thousands though, it's impossible to say.
I'm 
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