U.S. Threat Level
High. The Centers for Disease Control reported 48 states with West Nile virus infections and a total of 5,387 cases of the disease and 243 deaths in the U.S. last year. Cases have been on an exponential rise.

How It Spreads
Mosquitoes, blood transfusions, transplants, and breast-feeding.

What It Does
High fever, headache, abdominal pain, diarrhea, nausea, convulsions, muscle aches, rash, and vision loss. Neurological effects may be permanent.