Enlarge / A mosquito collected to test for mosquito-borne diseases. (credit: Getty | Jon Cherry)

A small town in Massachusetts is urging residents to stay indoors in the evenings after the spread of a dangerous mosquito-spread virus reached "critical risk level."

The virus causes Eastern equine encephalitis (EEE), which kills between 30 and 50 percent of people who are stricken—who are often children under the age of 15 and the elderly. Around half who survive are left permanently disabled, and some die within a few years due to complications. There is no treatment for EEE. So far, one person in the town—an elderly resident of Oxford—has already become seriously ill with neuroinvasive EEE.

EEE virus is spread by mosquitoes in certain swampy areas of the country, particularly in Atlantic and Gulf Coast states and the Great Lakes region. Mosquitoes shuttle the virus between wild birds and animals, including horses and humans. In humans, the virus causes very few cases in the US each year—an average of 11, according to the Centers for Disease Control and Prevention. But given the extreme risk of EEE, health officials take any spread seriously.