Plum Island is juuust off the long finger of Long Island, sitting there in the bay:
You can see that the community of Lyme, Connecticut is more like 3 miles across the water, nearby.
There are some old stories about the odd infected animal swimming it’s poor way across that water and making it to shore, then inland.
But nobody really knows.
Theres a lot of counter research going on that is just as and way more dangerous, than any weapons programs.