"We're in the midst of one of the largest West Nile outbreaks ever seen in the United States," said Dr. Lyle Petersen, a CDC official.
CDC: Alarming increase seen in West Nile cases →
CDC officials are also looking into the possibility that the virus mutated, but so far have no information showing that happened, Petersen said.
CDC: West Nile outbreak 'one of largest' in US →
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