"The lack of some of the factors that have been associated with more severe disease in previous pandemics, we are not seeing those." Testing for the virus is also becoming faster, Besser said. It appears that things are leveling off in Mexico." On in Mexico -- believed to be the source of the swine flu outbreak. -- The number of confirmed swine flu cases in the United States climbed to 286 in 36 states. "We have distributed test kits to every state and this will allow for more rapid diagnosis at the state level," he said.

"And that's encouraging information." In addition, some of the initial lab tests are heartening, Besser said.

Baldwin Besser, acting director of the U.S. And, while insisting that "we are not out of the woods," Dr. And, he added, "The situation in Mexico is encouraging. Centers for Disease Control and Prevention, said during an afternoon teleconference that "we are seeing a lot of encouraging signs." Among the encouraging signs. "So far the severity of illness we are seeing in this country is very similar to what we see with seasonal flu," Besser said.

But, the revised numbers indicate catching up on a backlog of lab tests, and not a sudden rise in new infections, federal health officials said.