One sure-fire way to tell if it's pollen or a virus is personal history. So if someone has seasonal allergies, then it could be the pollen, or an allergy to something else. Also, a virus is usually accompanied by a fever, not so with allergies.
I once had a student who was allergic to snow. Snow? Yes! Snow! I didn't know one could be allergic to snow so I asked her mom. She explained to me that as the snow thaws and refreezes, mold spores grow and disperse. So it's actually the spores that her daughter was allergic to, but it only happens when it snows. Go figure!
