Flu viruses evolve freshly somewhere in east or southeast Asia every year, spreading around the world over the next nine months before dying out, researchers report.

Genetic analysis by two teams of international researchers show that there are just a few initial sources of annual, seasonal influenza epidemics. The viruses spread around the world from these before dying.

Then every year, new strains emerge to infect people, according to the studies published in the journals Nature and Science.

One team led by Professor Edward Holmes of Pennsylvania State University cannot pinpoint the source but says both H3N2 and H1N1 strains of influenza appear to arise every year from a 'reservoir', perhaps in the tropics.

A second team led by Dr Colin Russell and Professor Derek Smith of the University of Cambridge analysed 13,000 samples of H3N2 flu taken since 2002.

These researchers demonstrate this source must be in east and southeast Asia, perhaps a different place every year.

"For over 60 years the global migration pattern of influenza viruses has been a mystery," Russell says.

Many experts have long believed Asia, and specifically China, to be the source of most influenza viruses.

Others hypothesised that flu viruses migrate back and forth between the northern and southern hemispheres, or that they cooked year-round in the tropics, to pop out every once in a while to the rest of the world, Russell says.

"We find that viruses come out of east and southeast Asia as a region each year and it is not any one particular country that is the continual source of influenza viruses," he says.

"So it is not as simple as saying out of China, because out of China is not the whole story."
Rainy seasons

In tropical regions, flu tends to break out in the rainy season.

"In east and southeast Asia there is a there a lot of variability in the timing of the rainy season and the timing of the epidemic," Russell says.

"Bangkok and Kuala Lumpur are only [1100 kilometres] apart but they have their flu epidemics at completely different times of year."

This means flu epidemics can be occurring almost year-round in Asia, he says.

Then the viruses die out every year in the Americas, Europe, Australia and the rest of Oceania, making these areas "evolutionary graveyards", Russell says.

Even if travellers carry the flu viruses back from the Americas to Asia, for example, people living in Asia are already immune to those particular variants.
How common is flu?

The World Health Organization estimates that annual influenza epidemics infect 5-15% of the world population each year, cause 3-5 million cases of severe illness and 250,000-500,000 deaths.

About 300 million people get the flu vaccine each year. Without it, says Smith, a person can expect to catch the flu about once every 10 years.

Smith says the findings are important for the experts who formulate the new flu vaccine each year.

It typically includes a cocktail of three strains, and the scientists try to predict which strains will cause the most trouble each year.

"If we are trying to predict what will happen a year from now we should be paying attention to what is happening in east and southeast Asia," he says.

The researchers say their study does not have any bearing on what might happen in a pandemic of a new source of flu, such as the H5N1 virus now circulating mostly among birds in Asia, Europe and Africa.

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.The influenza virus resides upon the wings of birds in the birds gut. In their annual migration they travel the world. The virus exists harmlessly in the birds gut. They are immune from the effects. Many birds do come from the orient but many more come from the south to north pattern and back again. Take for the artic tern they can travel up to 10000 miles every year to migrate to their nesting areas. I would say 90% of the earths water supply is from resivoirs or streams and rivers. All the waterfowl has do is deficate in the water. The bird flu is one that did originate in asia. Most of the orient hygene is non existant and direct contact transfers that strain of virus. Thats a bad thing because that virus has mutated into a new form to increase its survival rate. In the future I would expect many more drug resistant viruses, as we a now seeing in scattered cases and locations. The 1918 influenza didnt have just one area to start in it seemed to start everywhere world wide at once. There are still samples of this virus that exist in labratorys. The human population found a strain of flu that killed millions and only hope it doesnt happen again.

Yayyyyy....more fear mongering!!!!

Gotta love that.

It always brightens my day to know that there are several million things that could kill me at any time.

Somehow...that just makes life worth living.

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Air Cold, the blade stops;
from silent stone,
Death is preordained

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About 300 million people get the flu vaccine each year. Without it, says Smith, a person can expect to catch the flu about once every 10 years.

.....

It typically includes a cocktail of three strains, and the scientists try to predict which strains will cause the most trouble each year.

Alas catching the flu is a lot more common than that, but it's usually a mild form that is not covered by the flu shots. They try to cover the worst few strains in the vaccine, and admittedly since getting the injection each year (Paid for by work!!!) I have only had a couple of minor bouts of the flu, whereas my family have been unluckily enough so that one of them seems to get a nasty strain each couple of years, but at least they end up sharing it with everyone but me :)

The joys of school-age kids being sent to school sick as dogs by their parents, so that 1/2 their class end up getting sick as a result - argh!

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