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?

It's totally not appropriate. You need to explain your neighbour upfront, sure she'll understand. Also for next time, you can pre inform to the person who's holding your baby or you feel might touch the baby physically. Also, always keep a baby sanitizer along to hell in such situations