Eastern and western equine encephalitis virus (EEEV) are two of the most serious mosquito-borne diseases in the United States. While both viruses can cause serious illness, EEEV is more deadly. The viruses are spread by mosquitoes, which acquire the virus by feeding on infected birds. Humans and other animals can become infected with the virus if they are bitten by an infected mosquito. Symptoms of EEEV or WEEV infection include fever, headache, muscle weakness, confusion, and seizures. The illness can progress to encephalitis, which is inflammation of the brain. Encephalitis can lead to coma and death.