By any chance have you once been down with a cold virus? By chance did you heal from it? Of course you got over it! You cannot treat a cold directly, but your system usually creates resistance to a certain cold infection within a couple of days. We call that cured by your personal immune response!

Grownups have previously had a lot of the cold viruses. Parents have established immunity to the viruses they have had. So there are not numerous runny nose infections remaining for adults to become infected with. That is why parents only acquire a couple of colds each year whereas children get 10-12 runny noses each year.
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