There are eerie similarities between this latest outbreak of swine flu and the 1918 Spanish flu - the most devastating flu in the last 100 years.

There's the timing: Both started in late spring.

The age of the victims is roughly the same: Both viruses seem to target people between the ages of 20 and 40; not the very young and old, as is the case in the typical flu.

One more thing: In both cases, the flu is of the subtype H1N1.

When the 1918 flu first appeared in Kansas, during late spring, it was called the "three-day fever." People were symptomatic for three days, and then they recovered.

As spring turned to summer, the flu seemed to peter out.

Then, that fall, it came roaring back with a vengeance. Why or how is not known.

But by the end of the spring of 1919, the Spanish flu had killed more than 50 million people worldwide. In the United States, 28 percent of the population got the flu, and 2.5 percent of those people died.

In a typical flu season around 36,000 Americans die. Most flu viruses infect between 5 percent and 20 percent of the population, and less than 1 percent of those cases die.

If the swine flu were to play out as the 1918 flu did, around 2 million Americans would die.

That's unlikely to happen, however, because even if the virus were as virulent as the 1918 flu, modern medicine would "mitigate many - but not all - of these severe outcomes," said William Schaffner, an infectious disease researcher at Vanderbilt University in Tennessee.

For now, there is no way to tell whether the swine flu will die out this spring, or tarry through the summer and reappear as a stronger, meaner virus in the fall.

"Speaking personally," said Schaffner, "I think there is a likelihood this will re-emerge in the winter as a dominant flu strain."

He said it will probably die down in the next few weeks as the temperature and humidity increase.

"It would be unprecedented for a flu virus to maintain itself at strong levels during the summer months," he said.

Researchers caution it's still much too early to tell how this virus will shake out, and how the global community will respond.

"One thing we do know is that there is a real distinction between what is happening in Mexico and what is happening in the U.S.," said Schaffner.

There have been more than 170 deaths in Mexico, and only one in Texas. Four more deaths were confirmed in Mexico on Thursday.

Lab tests indicate the virus is identical in the two countries, said Schaffner. So, researchers are beginning to think that with so many more deaths in Mexico, there probably also are thousands of unreported mild and asymptomatic cases in Mexico, making this a fairly typical flu in terms of its severity.

"It could be acting just like a conventional outbreak," said Schaffner.

Or it could turn into something more virulent than we've seen in years.

"We've got a brand new virus," said Richard Olds, infectious disease specialist at the Medical College of Wisconsin. "We don't know much about it. How infectious is it? How virulent is it? Ask me in three days. In three days we'll know a lot more."