Allergy can last all year long. There are different problems at different times of the year. It starts with tree pollens after the first of the year. Next come the grass pollens, which may last until the first frost. Weed pollens start in the summer and they too last until the first frost. With all of the decaying vegetation and rain, molds become more of a problem in the late fall and winter. Of course, if you have a mold problem around your home, you can be bothered by molds all year long. We are inside more during the colder winter months and that is when dust problems increase.
The only place on earth that is said to be free of dust mites is at the Dead Sea. The feces from these microscopic creatures are easily airborne. Filters and meticulous cleaning help but it is impossible to completely get rid of dust mites and their feces. Animal danders are a real problem for some people. Cats are the worst offenders followed by dogs and birds. And there is no evidence that having a Chihuahua helps.