PARIS, Jan 27 (AFP) - World health officials have a store of evidence to back their warning on Tuesday that if bird flu mutated into a more contagious form it could kill millions of people.
That assertion may seem overblown to those for whom influenza is just a cold with attitude -- a bad case of the snuffles with fever, headache, coughing and muscular aches thrown in for good measure.
But the truth, say researchers, is quite darker.
Flu is a changeling, a survivor, a stealthy assailant which, in its most pathogenic strain, could rip around the world.
Indeed, the 1918-19 strain of so-called Spanish flu killed an estimated 40-50 million people: around twice the number of people who have died from AIDS, a disease that is now in its 23rd year.
Invariably, of course, flu circulates in a far milder form.
Even so, it can still endanger the young and elderly and people with chronic heart or lung problems or a pre-existing condition such as diabetes or kidney disease. The biggest risk is pneumonia, a common effect of flu infection.
Each year, "several hundred thousand people" die from flu, says World Health Organisation (WHO) spokesman Iain Simpson, the agency which sounded the alarm Tuesday about the killer potential if Asia's bird flu merged with human influenza.
Indeed, flu is such a problem that the WHO classifies it as a pandemic. It is a permanent health hazard that shifts from the northern hemisphere to the south and back again, in line with the respective winter there.
Rivals in the league of nasty diseases may grab the headlines for the lurid way they kill, but the real champion could be flu.
Unlike plague, cholera and malaria, this disease does not need poor hygiene or a tropic insect host to spread.
Unlike the AIDS virus, it does not need sexual intercourse, blood transfusion or shared use of drug syringes to be transmitted. Breathing in airborne, virus-laden droplets, coughed or sneezed by someone in proximity, is enough.
Unlike Ebola, which usually kills the patient within a couple of days, thus limiting its ability to spread, flu can incubate for several days before any symptoms are apparent.
That means it can spread swiftly around the world, because air travellers can take the disease to another country without even knowing they have it.
But flu's most feared characteristic is its ability to change.
Small shifts in its DNA code, called mutations, occur frequently in a viral strain. Usually, though, the change is so small that the new strain is not much different from the strain that circulated the previous year.
More troubling is a bigger change, involving a swapping of genes with other viruses.
This is why a mixture of bird flu and a common human influenza virus is so feared.
Bird flu (strain H5N1) is only transmitted from poultry to humans, but not from humans to humans.
On the other hand, it is extremely dangerous, with a mortality rate of 33 percent if the first outbreak of it, in Hong Kong in 1997 (six dead out of 18 cases), is a guide.
Combine that lethality with the contagiousness of the human flu virus, and the scene is set for a killer on a major scale. As an entirely new strain, no-one would have any antibody resistance to it, and time would be very limited for preparing and distributing a vaccine against it.
"The risk is of an adapted H5N1 virus against which humans have no defence, " virologist Sylvie van der Werf of France's Pasteur Institute said.
The H1N1 virus in the 1918-19 outbreak was so pathogenic that it could kill within 24 hours and, for reasons that are still mysterious, those most vulnerable were healthy young adults.
The emergence of a strain as nightmarish as this is very rare -- the last time was in 1968, when the 700,000 people were killed globally by the H3N2 "Hong Kong" flu -- but the risk is real.
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