Tw spoilers:
I don't know for sure but I heard on TikTok in one of the comments it was the water that infected them that makes a little sense cause how McCoy had tooken that shower first from getting the drink spilled on but I don't know cause we didn't see June, vonnie or Carmen get in so idk I think the reason why Ryan didn't get infected was because he couldn't swim and the end he couldn't bare too leave his friends behind so he jumped in afer