(CNN)Laurie Sylvia, the fourth person in Massachusetts infected with Eastern equine encephalitis (EEE) since the beginning of August, has died.
The other cases of infection this month are those of two men over 60 and another man between 19 and 30.
This is the first time EEE cases have been reported in Massachusetts since 2013.
State epidemiologist Catherine Brown told CNN in a statement that warmer temperatures and above-average rainfall in July sped up virus replication in mosquitoes, which could explain the uptick in EEE activity.
Migratory birds might have even spread a new strain of the virus, though lab tests that could prove that have yet to be completed, she said.
In Florida and Delaware, the virus has been detected in sentinel chickens, though neither state has seen any cases reported in humans.
EEE kills one-third of those infected
EEE is rare but potentially fatal. Anywhere from 5 to 10 human cases are reported every year, but about 30% of all cases are fatal, according to the Centers for Disease Control and Prevention.
The illness can cause brain swelling preceded by flu-like symptoms, including a high fever, chills and nausea. Severe cases could result in seizures or a coma that can cause brain damage, the CDC said.
The CDC recommends residents of impacted areas load up on repellant and avoid the outdoors after dark to lessen their risk.