It might take a week or so before he develops symptoms.   And then another week or two weeks after that before the symptoms become severe.   Or, he might not develop any symptoms at all, or have a mild case.   At any rate, we won't know the severity of his condition for at least a week or two.  He is 74, and mildly obese.  Age and obesity are two big risk factors for severity.

And, he didn't catch it because he didn't wear a mask.  He caught it because the people around him didn't wear masks.  Mask wearing does provide some protection to the wearer, but it's much more for the purpose of protecting other people.  

This does make Biden look a lot smarter for taking precautions.

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