Most people think of flu season as an amorphous block of time in winter, but it actually has a strictly-defined beginning and end. As of last week we’re officially in it—just in time for Thanksgiving holiday travel to make it even worse.

The Centers for Disease Control defines the flu season as starting when more than 2.4 percent of all outpatient doctor’s visits are for flu-like illnesses, which means we’re above the national baseline. As the season’s peak, flu-like complaints can make up around five percent of our medical visits, or even seven during a bad year. We’re only at 2.5 percent now, but that number generally starts jumping up quickly once we hit the holiday season.

For one thing, it’s colder now—being cold doesn’t make you get sick, contrary to popular belief, but spending more time inside means you’re more likely to share air with other people (and their germs). Chilly, dry air also keeps the flu virus alive better as it travels from person to person, and can irritate your mucus membranes and leave you more susceptible to infection. You’re also around more people during the holiday season: Friends and relatives may travel to you from areas with higher rates of flu infection than where you live. The general increase in travel also means the people you come in contact with from day to day—in your office, on your commute, at your kids’ schools—are more likely to get exposed to random strains of virus from distant regions of the country.

That’s why the CDC always recommends you get the flu shot early. Realistically, the best time is in September, especially if you’re in school or have kids who are, but the second best time is always right now. Today. Go to the closest pharmacy that takes your insurance (if you’re not sure they do, just give them a call beforehand) and get the flu shot. It won’t fully protect you until two weeks from now, so it won’t help you during the holidays as much, but every day you wait is another day you can get exposed.

Right now, if you’re a generally healthy person, you’re probably thinking you don’t really need the flu shot. You never really get the flu. You’re not going to be one of the people who gets hospitalized. You’ll be fine—you always are.

But setting aside the fact that literally thousands of perfectly healthy people go the hospital with flu infections and many hundreds of them die every year, you’re not really getting the flu shot for yourself. You can feel pretty much fine and still pass the flu virus on to people who are far more vulnerable than you are. Pregnant women who get the flu can have serious complications for themselves and their fetuses, and both immunocompromised and elderly people can get deathly ill from the virus.

And yes, it’s true that the vaccine isn’t that great. It’s a good year if it’s more than 50 percent effective. But even partial effectiveness can protect you from getting seriously sick if you do get the flu, and from passing it to too many people.

So go get your shot now. Even if you don’t think it’s that important, there’s no downside: You can’t get the flu from the vaccine, and it will help both you and everyone around you. Consider it a holiday gift to the whole country.