Summer allergies ofc. They cause a build up of mucus, if it's not being drained it will become infected. Presto tadaa, sinus infection. As a sufferer of year round allergies, I suggest a netti pot, particularly before an actual infection sets in. Not fun, but effective. Also, apple cider vinegar shots in the morning and evening. Again, not fun, but extremely effective, even after the infection has set it. The pollen=death star is completely valid.