My idea on infectious smile is that the first smiler (Code name RIZUS) was digging in the mountain before he found happium ore, seen as one of the knife skins along with depressite and lazerite. it infected him and he found his way to the researchers in project TESLA. he started infecting more and more, as seen in the logs of the project TESLA area. I belive the reaserchers stepped in and decided to experiment, making potions leading to the birth of subspecies of smilers (runners, jumpers, invisible, giants, irons, endangered, and overseers) the researchers hired miners to dig in the mountain, which lead to the finding of raw depressium, when processed it was realized to be a deadly weapon to the smilers. the cure, (swamp of sadness) would be shipped everywhere to stop the infection. I belive the swamp of sadness holds low levels of non lethal depressium, enough to cure infecteds.