In the wake of last summer’s outbreak of deaths and illness from mosquito-borne Eastern equine encephalitis in Massachusetts, a new study by Boston researchers concludes that the risk for severe complications or death among infected children is greatest when symptoms come on quickly with little warning.

Eastern equine, a rare and virulent virus that has been on the increase in the Northeast, is often fatal and there are no specific treatments for the disease. Care focuses on relieving the symptoms, including including brain swelling and seizures.