We are fairly certain it spreads by (you)s. But it's still debated whether it's you replying to an infected poster or the other way around or either or both (I had both when I got infected).
We also know there is an incubation period of a few posts, so after you get infected you won't show symptoms. It is not known whether you are contagious during this period.
We need some healthy people to experiment more. If any healthy people are reading this, try not replying to anybody who is infected and wait until an infected gives you a (you) to see if that spreads it. Or alternatively, reply to infected people but don't do anything to bait them into replying to you and see if that infects you.