West Nile virus still on the move.

• Position: YOUR LIFE

More than 14,000 Americans have been diagnosed with West Nile virus since 1999 when it first was discovered in the Northeast. It now can be found coast to coast. Although the current fatality rate remains relatively low, the virus' ability to spread through organ transplants, breast milk, and blood transfusions has added a new level of complexity to this health problem.

West Nile virus and encephalitis viruses come in many forms, but have two things in common--a nonhuman primary host and an infected blood-feeding arthropod, such as a mosquito or tick, reports Fabio Del Piero, assistant professor of pathology at the University of Pennsylvania, Philadelphia. Del Piero says there are five main arboviral agents of encephalitis that are transmitted by mosquitoes: West Nile, eastern equine, western equine, St. Louis, and La Crosse. Another virus, Powassan, is a minor cause of encephalitis in the northern U.S. and is transmitted by ticks. Most encephalitis cases occur from June through September, when these pests are most active.

A majority of human infections are asymptomatic or result in flu-like symptoms. In a very few instances, infection may lead...