Colds are caused by a virus or bacteria, period. Flu is caused by a virus or bacteria, period. There is no opinion or argument, that's the scientific fact. Yes, if your immune system is lowered due to fatigue or a recent sickness you are more susceptible, but walking in the rain isn't going to hurt you unless someone sneezes in your face.
However, the original problem in this thread makes me mad too. I tell all my dcMoms to carry wipes in their purses and wipe their children's hands before getting out of the shopping cart or off the bus, but before putting their child in the carseat or home. Washing hands is the first line against germs and not touching your face. I remind them and remind them. I bet none of them do it.