Last week, a neighbour kissed my baby on his lips. I was so surprised, I didn't even respond. I think it's unhygienic and disgusting. Am I overreacting? How can I be polite, but make sure this doesn't happen again?

I don't think you are overreacting at all. If my neighbour kissed our son on the lips, I would react the same way, even if they were close friends. The next time the neighbour is with you, I would bring it up and ask her to give kisses only on the cheek or forehead. I think showing affection is important and depending on the child's relationship with the neighbour a kiss could be beneficial. But if you don't want that, just ask the neighbour (or anybody who intends to give a kiss) not to do it anymore.