In the movie twilight what flu was going on?

The flu going around in the movie Twilight isn't a specific type of flu, but rather a generalized illness used to explain why Bella Swan is feeling under the weather and is therefore homebound.

This allows her to encounter Edward Cullen and the rest of the vampire family, setting the plot in motion. The illness is never explicitly named, but it's clear that it's not a serious ailment and Bella recovers quickly.