Many people are concerned about germs but I learned in one of my medical terminology classes that it is possible to pass many diseases such as flu, cold, and even herpes to a baby in this manner and this is why some doctors will recommend to parents not to let people kiss the baby's face. People can be contagious whether or not they know it and whether or not the parents of the child know. I shudder when I see people/adults kissing kids on the lips.
Think about how often babies rub their faces with their hands or put their hands into their mouths. We have several mucous membranes on our faces and babies commonly rub their faces and could get disease causing germs/matter onto their hands which are often put in their mouths, nose, or eyes. If I was a mother, which I am not, I would mention to those holding my child to kiss the baby on the high on the top of the head or not at all for this reason. And if they have a problem with this then they don't have to have such close contact with my child. The child's health is of the utmost importance and the parents must do their best to be informed and inform others.