I think it has to do with their (incorrect) assessment of the situation. Rather than reevaluate their assessment (as that would increase their anxiety), they rationalize your assessment as overreaction.
Yet another example of the bullying culture of the US.
It is not an airborne infection.
Only in the sense that "airborne" is a scientific medical term meaning larger than 0.5 µm. When most people hear "airborne" they think it means "in the air", which the virus certainly is. The use of this terminology in the strict scientific sense when speaking to general audiences is borderline irresponsible.