Sept. 22, 2012, 10:20 p.m.
More than 130 people have died in the US as a result of West Nile Virus, the disease that is spread to humans by mosquitoes.
This year's outbreak looks set to be the worst on record.
With no available vaccine, Americans currently have little protection. Scientists say people may eventually acquire immunity through exposure, but that could take several decades.
Jane O'Brien reports.