Only certain types of mosquitoes carry viruses of concern to people. A mosquito must first become infected by feeding on a bird that has the virus, then bite a human or animal to pass the disease along. There are five main types of viral illness carried by mosquitoes in the United States.

West Nile virus: WNV has been reported in Mississippi in horses, birds, mosquitoes and humans. In humans, WNV infection can lead to dangerous conditions such as meningitis or encephalitis. Anyone can get WNV, but people over 50 years of age are more severely affected. Approximately 3 percent to 15 percent of people with WNV die. Unlike other mosquito-borne illnesses, certain species of birds (especially crows and blue jays) can also get sick and die from the disease, as can horses.

St. Louis encephalitis: SLE causes illness only in humans. Historically, it has been the most common disease carried by mosquitoes in Mississippi. SLE primarily affects the elderly, occurring in summer and early fall. SLE rarely causes death, although approximately 15 percent of the patients with severe symptoms die.

LaCrosse encephalitis: LAC can cause illness in humans, though it is more common in the midwestern U.S. rather than Mississippi. Cases occur from spring to fall, usually affecting children under 16 years of age. Only about one percent of people who become ill with LAC die.

Eastern Equine encephalitis: EEE causes illness in humans, horses and some birds. Very few human cases of EEE have ever occurred in Mississippi. Most infections occur in horses, from spring to fall. People of all ages can get the disease, but children are more likely to have severe illness or die. Approximately 30 to 60 percent of those who become ill with EEE die.