Massachusetts officials have raised the risk level from the dangerous Eastern equine encephalitis virus to "critical" in some towns and say the threat from mosquito-born illness is the highest in decades.

Separately, health authorities said on Wednesday that the state's first human case of the virus for 2012 had been identified, although most likely contracted out of state.

A man in his 60s from Middlesex County, west of Boston, became ill on July 28 after having recently traveled to the Mid-Atlantic region, where he reported getting numerous mosquito bites, the state Department of Public Health said.

The man was hospitalized and released.

The potentially deadly EEE virus is spread to humans through the bite of an infected mosquito.

Severe cases begin with a sudden onset of headache, high fever, chills, vomiting and lethargy, and can progress into disorientation, seizures and coma, according to the U.S. Centers for Disease Control.

Inflammation and swelling of the brain, called encephalitis, is the most frequent serious complication.

State officials said a mild winter in the Northeast United States contributed to higher mosquito populations this summer in Massachusetts and potentially neighboring states, and is also why mosquitoes carrying EEE were found earlier than normal.

Aerial spraying - conducted in about 20 Massachusetts communities in late July - will likely resume early next week in six towns, said state Department of Public Health spokeswoman Anne Roach.

Bodies of water with mosquitoes testing positive for the virus have been found multiple times already this summer, concentrated in southeastern Massachusetts.

The risk levels in Easton, Raynham, Taunton and West Bridgewater, about 40 miles south of Boston, are now considered "critical." Two other nearby towns had their risk levels increased to high from moderate.

Positive findings for the virus have been found in other parts of Massachusetts but not to the degree seen in the southeastern area, Roach said.

Massachusetts had two cases of EEE, one fatal, in 2011.

West Nile Virus, another serious medical condition usually contracted from the bite of an infected mosquito, is also on the rise in 2012. Human cases reported nationwide by the CDC through the end of July was the highest since 2004.