Swine flu is entering the mixing bowl. The virus that emerged this spring in Mexico is now reaching the place where it may have its greatest potential to mutate—the tropics. Cases have recently been identified in India, Vietnam and the Philippines.

Flu viruses circulate year-round in tropical regions, where they can mix with each other, mutate and spawn new strains. Two studies published last year outlined this process, showing how seasonal flu strains in the temperate regions originate in tropical regions.

Unfortunately, as Declan Butler at Nature reports, surveillance networks are often weak in tropical countries, so it may be difficult to monitor the evolutionary trajectory of the new H1N1 virus.

Vaccine experts breathed a sigh of relief when the first sequences of the new H1N1 virus revealed relatively little variability among isolates. That makes it easier for manufacturers to produce a vaccine that could work against all forms of the virus. But the virus may surprise us in the fall, popping up as a different sort of beast—or beasts.

There are also fears that the virus will wreak havoc in sub-Saharan Africa, where there are many people living with HIV, weakening their immune system and possibly increasing their vulnerability to swine flu.