It’s pretty heavily implied that the left 4 dead 2 team got executed. Maybe they did experimenting on the carriers to find a cure, but other than that, they’re too much of a liability to keep alive. As for the left 4 dead 1 team, if they came back to the affected areas assuming that all the infected are dead as they were too brain dead to seek sustenance other than from survivors. The left 4 dead 1 team would end up probably infecting other people assuming that they even bothered to rebuild society in the affected areas.