Here's the good news: The swine flu that is busy spreading across the globe may soon be headed for a summer vacation.

The bad news: There's a distinct possibility it could return with a vengeance in the fall.

The hope: By the time the swine flu can make an encore, we'll have a vaccine ready to take it on.

"Flu doesn't do well in the summer. The viruses don't like the heat," said Rachel Schwartz, a researcher at St. Louis University's Institute for Biosecurity.

"The swine flu will likely drop off in the summer. If it returns in the fall, it will likely be stronger."

Flu viruses constantly mutate. That's why we need a shot of new vaccine every year.

And seasonal flu viruses routinely subside during the summer and return in altered form in the fall. But this flu, which combines genes from flu viruses found in pigs, birds and people, is unlike any previous seasonal viruses.

That has scientists looking for historical precedents to help understand what could be in store. What has them most concerned is the flu pandemic of 1918-19.

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