Maybe it's not explained?
A user on the wikia posted this theory in the Talk section of the Gas article:
It is interesting to note that, according to Elijah's notes on the Cloud sample in his chemistry set, the Cloud grows or propagates in some manner which necessitates the removal of the residue at one jar worth a day. This may lend some weight to the thought that The Cloud is some sort of mold, fungus, or bacteria that was released from the confines of the foundations. It is a rare and not widely-known risk of salt and other types of mining in the real world already. Given how little we know about The Cloud and the Ghost people, it really is just speculation, interesting as the idea may be.
This opens another line of thought that the Ghost People could be either hosts to a parasitic or symbiotic infestation that grants them a limited regenerative ability, or more akin to Spore Carriers. Either one offers an explaination for dragging others to The Cloud as a means of reproducing and spreading out. Very interesting possibilities.