WRAPUP 4-Flu spreads in U.S., world, winds down in Mexico

(For full coverage of the flu outbreak, click [nFLU])

* U.S. officials say outbreak "on the upswing"

* Mexicans take precautions against infection

* First genetic analysis shows virus mutations

(Updates throughout with new reports on flu, US figures)

By Maggie Fox, Health and Science Editor

WASHINGTON, May 7 (Reuters) - U.S. health officials predicted worsening outbreaks of the new H1N1 flu on Thursday, and a top global health official predicted up to a third of the world's population could eventually become infected even as Mexican officials said the worst was over.

A batch of detailed studies on the new swine flu virus showed it was a strange marriage between a triple-hybrid virus with pig, human and bird elements and a European swine virus not seen before in North America.

But some pork bans appeared to be easing and U.S. agriculture officials promised unspecified help to farmers.