West Nile virus is transmitted to humans through the bite of an infected mosquito. West Nile virus cannot be prevented through vaccines or directly treated through medication. Although most people (8 in 10) who are infected do not feel sick, about 1 in 150 develop a serious, sometimes fatal, illness. People over age 60 are at greater risk for severe illness if they contract the disease. Symptoms in mild cases include fever, headache, body aches, joint pain, vomiting, diarrhea, or rash. Symptoms of more severe infections may include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, muscle weakness, convulsions, vision loss, numbness, and paralysis. Persons with these symptoms need to seek medical attention immediately.