If you trace back the spread of Lyme disease in time, you get two points. One in Connecticut, and one on Long Island, where workers got on the boats to Plum Island.
The lab was studying diseases similar to Lyme disease at the time.
All those are facts.
The conspiracy theory is that Lyme disease was accidentally released by that lab.
https://en.wikipedia.org/wiki/Building_257#Discredited_consp...