Why are the infected sims back?

I sent a sim to Strangerville because I was getting infected sims popping up in my other neighborhoods and she went thru the whole process and defeated Mama Plant and now it is dead.  She returned home and I assumed all the infected in all other other neighborhoods would now be cleansed.

But today,  not a week later I see another one of those infected robot types geeking around.  My sim still has plenty of the vaccines she made to kill Mama so does she have to go and hunt down this guy and toss it on him too?   I think that's pretty bad programming if the game left that big a loop hole.