Assuming you're taking about Eastern Equine Encephalitis, it's carried by the Black-tailed Mosquito, Culiseta melanura. Interesting thing about this mosquito species is it has very specific requirements for it's breeding/egg laying site. The female needs acidic water with a PH lower than 5, she finds it in bogs and low swampy areas. But it doesn't stop there, she lays her eggs in what are called "crypts", small holes filed with water at the base of live Atlantic White Cedar, Red Maple and rotted trunks. This mosquito does not breed in a bucket of water in your backyard. I have a friend who's job it is to monitor their populations, I've been out with him in a cedar swamp, he uses a very small aquarium dip net to fit into holes often 3-4" wide to catch the larvae. If he finds larvae in one of these crypts it's pretty much guaranteed to be the target species. That's why the disease is so rare, even this year where ground water levels are high in New England following two relatively rainy summers (2018/2019), the mosquito is doing well but there are only a handful of human cases.
-AJ