Love this article:

http://www.scarymommy.com/kids-with-allergies/

We were at a birthday party one and this little girl was totally all up in Xander's face and just wanted to keep planting him with kisses, but she was eating cake and drinking milk, so I had to be that mom who kept restraining her and keeping her from causing my kid to have an allergic reaction to her mouth!

That's another reason not to let your kid go up and hug/kiss other children they don't know - you never know what allergies people might be dealing with!