| 20:11 PM, Friday, 26 June 2009
The sun has been shining, the temperature rising, and it really does feels like summer. So not the traditional time one would expect a flu virus to flourish.
UV light cooks the viral goose pretty quickly, and people are outside more rather than being huddled together swopping germs (let's not spoil this theory by mentioning air conditioning or public transport).
But the H1N1 swine flu virus is not like seasonal flu.
It's sufficiently novel and contagious that it is continuing to spread despite the warmer weather. Let's look at what's happening in the UK and USA.
The number of cases of swine flu in the UK has doubled in a week. Laboratory confirmed cases stand at 4322. England saw its biggest daily increase today with 535 new cases, most of them coming because of a surge in London and the West Midlands.
Both areas now have more cases than the whole of Scotland, where the virus continues to flourish in the Glasgow area.
With the virus now very firmly established in London it's likely that further big increases will follow. And remember that many cases are going unreported.
Does that mean a peak in cases - a full-blown national epidemic - will come earlier than expected, perhaps in late August or September? Possibly, but the expectation is that once the school holidays begin there could be a pause with a peak coming from autumn onwards.
Then we could see tens of thousands of cases a week and every part of the UK will be affected. At present, in Wales, north-east and north-west England, there's still hardly any flu at all, so the virus is still not fully embedded here.
Now to the United States. The CDC in Atlanta has given another of its very helpful briefings to journalists.
Dr Anne Schuchat set out the situation there: 27,717 laboratory-defined cases with more than 3000 hospitalisations and 127 fatalities.
"This virus is not going away" she said, "and the reported cases are the tip of the iceberg. We are estimating there have been at least one million cases in the United States - not perfectly accurate but a ball-park figure."
That estimate is based on some telephone polls asking people whether they'd had flu-like symptoms in the past three weeks (not the most accurate way of estimating illness) and some community surveys which suggested 6% of people in hot-spot regions may have had the virus.
While not terribly accurate, the CDC appears confident that at least one million Americans have been infected.
If so, that is very reassuring news because it means the fatality rate is really very low. North America is about a month or so ahead of the UK with its outbreak so what happens there is a good guide to what may happen here.
So who is getting H1N1 swine flu in the US? Dr Schuchat said the virus was affecting mainly younger people rather than the elderly and made these points:
1.Nearly 80% of those hospitalised have been under 50
2. Average age for hospitalisation is 19
3. Average age for those who've died is higher, at 37
4. About 75% of those who've died have had underlying health conditions
5. There've been very few cases in people over 65, so they are very unlikely to be infected, but when they do get it, elderly people have a higher risk of complications and death
So how mild is mild and what sort of symptoms do people have? Dr Schuchat said most had a fever and respiratory symptoms, a cough, a cold, with some having diarrhoea. People may be miserable in bed for a few days, but the illness clears up on its own.
The US, unlike the UK, has restricted antivirals to those at most risk of complications, so comparitively few doses of Tamiflu have been handed out.
The CDC can't say yet how many people are asymptomatic - ie they have such mild illness they don't even realise they've had swine flu, but with a million plus Americans thought to have had swine flu, it must be a pretty sizeable number.
But a minority can have a much tougher time according to Dr Schuchat: "It's important for everyone to be aware of the virus, and especially for those with underlying health conditions, asthma, diabetes, chronic lung disease, and pregnant women: these people need to be especially concerned if they develop a fever and respitory illness."
Some mention has been made of obesity being a risk factor.
Dr Schuhat said that those who were obese, and especially the morbidly obese (those with a BMI - Body Mass Index - above 40) often have chronic lung disease and that's why they were at risk of complications. Being obese on its own does not seem to be a risk factor.
To sum up, swine flu is here to stay. For the vast majority it is nothing to worry about. For a small minority it can cause serious illness. It should neither be dismissed out of hand nor be a reason for panic.
Scientists must continue to monitor the virus to see if it mutates or becomes more virulent or becomes resistant to antivirals - thankfully there's no sign of either at present.