It’s fall. You have a runny nose, maybe a headache. Your first thought: It’s a cold. Your second thought: It’s the flu. Truth is, it might not be either. It could be allergies.

About 10 percent of the U.S. population suffers from ragweed allergies each fall. Ragweed is the most common culprit for fall allergies—it’s responsible for about 90 percent of all pollen-related allergies in the country. And no wonder it’s such a problem for so many: In a single season, one plant can produce a billion pollen grains, which are light and travel easily. In fact, pollen can travel as far as 400 miles on the wind!

Fall allergy symptoms are similar to those of spring sufferers, including sneezing, runny nose, itchy or watery eyes, itchy throat, headache, sinus pain and pressure, difficulty sleeping, and increased asthma symptoms. Ragweed can also cause hives on the skin. Over-the-counter medications, prescriptions, and immunotherapy can all provide relief; talk to your doctor about the best option for you.

To protect yourself, stay indoors when pollen counts are high, and keep your windows closed at home and in your car. Change your clothes after you’ve been outside, and try showering when you come home, to get the pollen out of your hair and off your skin.

Ragweed can be found all across the country, though it’s most common in the East and Midwest. Ragweed season used to start in mid-August and run through September. Now, however, it is beginning earlier and lasting longer, possibly due to climate changes that are extending its growing season. So you can count on having the sniffles a little longer than usual, too.