When the trailer checks in with Billy, he's not only living it up as a lifeguard — strutting around shirtless, flirting with a few different girls — but there's also a shot of him in the shower. He has a deeply troubled look on his face and we soon see why: there appears to be some kind of red and black growth blossoming on his left forearm.
So, what could this mean? The poster and previous teasers for the season included rat imagery and footage of rats running through a warehouse. Rats are known for carrying disease, right? What if the rats living underground in Hawkins got into those tunnels that Hopper and the kids were wandering around in under the pumpkin patch in season two and are now carrying an infectious, Upside Down-borne illness? The infection on his arm is in the same red and black shades as those gross, slimy vines that are all over the Upside Down, so it checks out.
Obviously, we can't be sure of how the rats came in contact with Billy, but it wouldn't be that hard to imagine they somehow spread the disease (if it IS a disease) to the pool.
