There have been no cases of the mosquito-borne West Nile virus so far this year in Nebraska. An insect-transmitted illness expert at Nebraska's health department told the Lincoln Journal Star that it's unusual to have no cases by this time. Disease scientist Jeff Hamik says a cool spring might have delayed mosquito breeding, resulting in the longest period without a pool of West Nile-positive mosquitoes in Nebraska history. But Hamik says West Nile cases will start cropping up soon. Most people with West Nile virus don't have symptoms. It can cause flu-like symptoms and sometimes leads to severe illness.