| 15:34 PM, Thursday, 11 June 2009
Flu pandemics are an inevitable fact of life.
There were three in the 20th Century. The worst, Spanish flu in 1918, killed perhaps 50 to 100 million people - but that was before the antibiotic era.
And H1N1 flu is currently so mild that, in terms of deaths, it might not be much worse than a bad normal winter of flu.
But it is different in the groups it targets. Very few elderly people are catching H1N1 flu, probably because they have immunity due to exposure to similar flu viruses.
Instead, the peak groups are children and young adults aged 5-24, followed by children under five, and then adults under 50. Most of those who fall seriously ill, with complications like pneumonia, have underlying health problems.
But of the 140 or so deaths so far worldwide, about half have been in previously-healthy people. That is what prompted the World Health Organization to change its description of the virus from a mild to a moderate disease.
People will want to know when they are likely to be exposed to H1N1. That's impossible to say with any accuracy, but the summer months may suppress the virus somewhat in Britain, and it may be the late autumn or beyond before we see a full-scale epidemic in every community.
The government's pandemic plans suggest that perhaps a third of the population could get infected, causing huge rates of absenteeism from work, and extra pressure on the health service.
There is concern that the virus might mutate in the southern hemisphere over its winter and become more virulent, but there's no sign of that yet.
A pandemic vaccine is in early development; it will be autumn before the first doses are ready; they will be earmarked for front-line health workers and those with health problems.