Authorities say a man in his 70s has died from the virus. It's the 10th death in the United States this year.
The rare mosquito-borne illness has been detected in mosquito samples in several states, including Pennsylvania, New Jersey and Delaware.
The Pennsylvania departments of health and agriculture issued a warning on September 21 after confirming eastern equine encephalitis in Erie, Carbon, and Monroe counties. The virus was found in pheasants, horses and a wild turkey.
The CDC says nearly a third of human cases are fatal, and many survivors are left with permanent neurological damage.
The virus is carried by birds. Mosquitoes can transmit it to humans, horses and other birds.
Officials say this is the worst outbreak of EEE since the 1950s.
Officials are urging people to avoid mosquito bites when outdoors by wearing mosquito repellent and covering exposed skin with lightweight clothing.
Homeowners should also make sure window screens are in good condition and eliminate standing water around their properties.
-- The Associated Press contributed to this report.