Winter usually comes with perpetual cold — not just outdoor temperatures, but also within our bodies. As soon as the arctic weather sets in, people start coughing and sneezing. It’s not supernatural; there’s a reason you feel endlessly ill in December and the other winter months.

It’s cold

Unfortunately, it turns out our nagging mothers were right when they yelled at us to wear more layers. Researchers have shown that the most common viruses that cause colds in humans replicate better in cool environments, which means they prefer our noses to our warm, gooey guts. But the cooler air also makes us weaker to defend ourselves — when it’s cold out, according to Popular Science, the immune system produces fewer virus-fighting agents, so the infection can just traipse in like it owns the place.

“Cooler temperatures meant a more sluggish immune response and a greater susceptibility to infection,” PBS reported.

It’s dark

For a lot of people in the winter, especially the more north you live, the sun is coming up as you’re heading to work and sets before you leave the office. That means you are not basking in its rays as much as you do in other months, and your skin produces less vitamin D as a result. And that vitamin may be playing a role in your body’s defenses. Although the evidence is not conclusive, the Huffington Post cites research suggesting that vitamin D helps prevent upper respiratory infections.

Read: Why You Need Vitamin D, And 3 Ways To Get Strong Bones

Among its many applications, vitamin D is involved in immune function, as well as cell and bone growth and the body’s calcium absorption. It has also been linked to mental health.

It’s everywhere

The law of odds tells us that the more people around you who are sick, the more likely you are to catch something. And we must remember that the people around us who are careless about spreading germs in warmer months probably don’t suddenly start washing their hands more in the winter. The Centers for Disease Control and Prevention warn that germs in droplets expelled during coughs or sneezes can live on surfaces like doorknobs, desks and refrigerator handles for two hours or even longer. Person A coughs into their hand, picks up the coffee pot. Person B touches the coffee pot, scratches the corner of their mouth. And so on.

The CDC also notes that with the flu in particular, people may be contagious before they even know they are sick — and thus are not taking precautions to avoid infecting other people. And that infectious quality has a long reach: “People with flu can spread it to others up to about 6 feet away.”

Read: The Flu vs. The Flu Shot