The United States is in the midst of the largest outbreak of West Nile virus since that virus was first discovered here in 1999. So far this year, more than 1,100 cases of human West Nile infection have been reported, about half of them in Texas, and at least 40 people have died from the virus. More cases are expected, since West Nile virus infections generally peak in late August and September. The virus is transmitted to humans by mosquitoes. Mosquitoes pick it up by feeding on birds carrying the virus. Very rarely, West Nile virus infection occurs due to transfusion of infected blood. It can’t spread from human to human by casual contact. None of the antiviral drugs currently available kill West Nile virus. And no vaccine is available. Prevention is best: stay inside when mosquitoes are most likely to bite, and use insect repellant when outside.