Is my 'illness' regional?

So yesterday, as usual, I drove down to Gloucester to visit my mother.

The whole time, I was encouraged by the fact that I did not cough once and was actually quite sprightly.

Driving home, I passed through Ipswich, turned a corner, and immediately started coughing and hacking. By the time I got to the cottage, I was feeling lethargic and ready for a nap.

I don't know what could possibly be that localized, but I do know that my sister (the one that lives in Gloucester) frequently has to take allergy meds when she comes up here.

It can't be the flora because I see the same old plants all over the place.

So what is it?