RSV vs common cold in infant

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For the mamas who have experienced their infants having RSV and a common cold…. Or know anything about the sickness:

What made you suspect RSV vs a common cold?

My son is three weeks. His sister is almost two. We just had the the stomach bug all at the same time and she also become stuffy. Well my son is now congested too. He seems to get more phlegmy when he eats. No fever or anything else. I’m just anxious of what it could turn into.

I just dread RSV. It sounds so terrifying.