Carried by birds and transmitted to humans through mosquitoes, West Nile can cause serious problems, especially in people who have weak immune systems.
Over the years, the virus has seemed to wane, causing fewer outbreaks and less sickness. But this year, that's changed.
According to the Centers for Disease Control, so far this year, 43 states have reported West Nile virus infections in people, birds or mosquitoes.
The total number of human cases is now about 700, including more than 25 deaths since January. Over 80 percent of the cases have been reported in six states - Texas, Mississippi, Louisiana, Oklahoma, South Dakota and California.
The CDC recommends people pay attention to mosquitoes and their breeding grounds. These bugs love water and lay their eggs in standing wet beds, such as a pet's water dish, old tires and flower pots.
Make sure standing water is removed. Also, avoid the early morning and early evening hours when mosquitoes are out. In higher risk areas, if you must leave home, wear long sleeves and long pants to avoid being bitten.