Even as the swine flu's global progression slows, experts said on Tuesday the world must brace for a second wave of infection that, previous pandemics have shown, could be far more virulent.
All three of the major flu pandemics of the 20th century, including the Spanish Flu of 1918, which left at least 40 million dead, started with a milder outbreaks in the northern hemisphere's late spring, they point out.
Pandemics typically begin with a "'herald wave', heralding something else coming along," said John Oxford, a top virologist at Saint Bartholomew's and the Royal London Hospital.
"It is disconcerting that in 1918 there was a summer outbreak that was fairly mild. It should have been a warning for the big wave, that came in the fall and winter," he told AFP by phone.
The new strain of virus emerged in Mexico, where it has infected nearly 800 people and killed 26, according to government figures.
Worldwide, the swine flu has spread to 21 countries on four continents, according to the World Health Organisation (WHO).
But even as Mexico City planned to ease an emergency that has paralysed the capital for five days, a top health official there warned that a resurgence in winter was "predictable." WHO chief Margaret Chan, meanwhile, expressed her fear that the flu could return "with a vengeance."
"1889, 1918, 1957 and 1968 all saw a circulating new flu strain in the spring and early summer of the year, just at the end of the influenza season across the northern hemisphere," George Gehner, a historian at Wichita State University and expert on swine flu, told AFP.
A "second wave" is highly likely, but there is no guarantee that it would be more lethal, both Oxford and Gehner said.
And even it did prove more virulent, the world is far better equipped today than a century ago to handle a killer flu.
"We will have totally failed in our preparation for this if we get a 1918 scenario," said Oxford. "We've got antiviral drugs and the knowledge base about transmission, and also soon vaccines, I suspect."
Antibiotics, unknown a century ago, could also save millions of lives. Some sixty percent of deaths in 1918 were due to secondary, bacterial infections resulting in pneumonia and other respiratory disease.
How the current swine flu develops in the southern hemisphere's coming winter, and whether the elderly count among its victims so far in Mexico, will both provide important clues in the coming months as to what may lie ahead.
"For the 1918 pandemic, it was the October-November wave that came back from the southern hemisphere that was so much more deadly," commented Patrick Berche, head of microbiology research at the Necker Hospital for Children in Paris.
What health experts fear most is that the virus will continue to mutate, mixing its genes with other flu viruses present in birds, pigs and humans.
Many scientists believe that the 1918 flu evolved into a strain that "tricked" the human immune system into attacking lung tissue, according to Oxford.
What happens below the equator "will give us some fair warning about what might happen here," he said, speaking from London.
Another uncomfortable parallel between past and present is the profile of the victims. While relatively few in number, many of the deaths in Mexico were among young adults, as was true in 1918.
Most seasonal flu fatalities, which number 250,000 to 500,000 per year, occur mainly among the very young and very old.
Health officials in Mexico are still piecing together data on those infected and those who died.
"I will be very surprised if there were no elderly deaths," said Oxford. "It would be good news, because this is usually a vulnerable group."
If persons over 60 escaped largely unscathed, it could also mean that they have some residual "cross immunity" from previous pandemics, notably in 1957, he said.