Why do complete strangers feel the need to come up and touch my kids?

Im not just talking about elderly people but it seems like EVERYONE and their momma feels the need to come up and touch my children. expecially my 3 month old. she's unusually small because she was a preemie but people KNOW how easy it is for babies to get sick so why do they feel the need to put their grubby little paws all over my kids