I think a lot of these comments are completely out of context with the OP, which is about a young mother complaining about her mum's neighbour stoking her child's cheek when she visits. She's not a complete stranger in the street, she's someone her mother knows, and probably she herself knows. It's a sad world we live in when you can't show a little affection to your neighbour's grandchild, with a little stoke, or a playful tickle.
I totally understand people's concerns about germs, especially with very young babies, but the baby mentioned in the OP was 10 months old, probably crawling everywhere and picking up goodness knows what off the floor and putting it into their mouth. Is a stroke on the cheek from a friendly neighbour really so dreadful?!
I think most of us wouldn't condone complete strangers simply coming up to a pushchair and holding hands/stroking cheek, but if the child is already interacting with someone, say, in a queue, or on the bus, I fail to see what harm is done ... germs are going to be on buses, or supermarket trolleys, etc., there's no way of getting away from them, and it's how babies build up their immunities ... by being exposed. So, the odd cheek stroke, I'd have no problem with.