Can cold weather make you sick? Your grandma wasn't entirely wrong.

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Grandma’s warnings about catching a cold walking barefoot on a chilly floor or going outside with wet hair have some truth.

Colder temperatures, especially in winter months, won’t cause a common cold. But they can weaken your immune system, and colder, drier air can help respiratory viruses thrive.

“That’s actually not an old wives’ tale,” Dr. Winston Rajendram, a family medicine physician at Northwestern Medicine in the Chicago suburbs, told USA TODAY.

Those crowded holiday parties also can be superspreading events.

“We find in colder temperatures, the viruses are more active,” he said. “And then there’s a higher risk of catching the virus during colder temperatures.”

Why are viruses more active?

Viruses need a host to replicate, and they can survive outside only briefly.

When people sick with a common cold or COVID-19 cough or sneeze, they let out respiratory droplets containing the virus, said Andrew Pekosz, a professor of molecular biology and immunology at Johns Hopkins Bloomberg School of Public Health. If the air is cooler and drier, droplets stay in the air longer, which increases the chances of infecting people nearby. That also happens with air conditioning, which can reduce humidity.

“That opens up a wider window of time that you could potentially get infected if you walk into a room, or if you touch a surface that someone has just contaminated with these viruses,” Pekosz said.

With higher temperature and humidity, viruses can’t survive outside a host as long. Research suggests flu viruses in high humidity can cling to water molecules and then fall out of the air. That prevents viruses from staying airborne.

People can get sick from COVID-19 and flu year-round, including in tropical regions, Pekosz added, but there needs to low enough humidity for transmission to occur.

“It just moves the needle a little bit more in the direction that favors a virus, as opposed to favoring us,” he said.

What about feeling cold?

Cooler weather can dampen the immune system, which makes our bodies more susceptible to infection. 

The body is generally good at responding to drops in temperature, but the nose and upper respiratory tracts can respond more slowly, Pekosz said.

That could explain why we get sick with these kinds of viruses from our respiratory tracts, as one study examining immune responses in naval cavities’ cells found.

But we go inside in the winter, away from the cold

In cold winter months, we spend more time indoors, often with groups of people. For example, think of eating lunch at work in the summer versus the winter.

Viruses spread more easily inside, because air flow and turnover is not as fast compared with the outdoors.

“Those viruses tend to hang around a little bit longer, and you stay in those spaces for a little bit longer,” Pekosz said. “Those two things together help explain some of why we see these winter surges with viruses like influenza, SARS-CoV-2 and RSV.”

What can we do to protect ourselves?

Rajendram recommends people talk with their physician about vaccines during checkups.

He suggests everyone ages 6 months and older get the flu shot. Last flu season in the U.S., there were 470,000 flu-related hospitalizations and 28,000 people died from complications related to the flu, according to the Centers for Disease Control and Prevention. Vaccines can help reduce risk of serious infection.

The same is true for COVID-19, which has an updated vaccine for everyone ages 6 months and older, as well as respiratory syncytial virus, or RSV, with vaccination recommended for elderly people, pregnant women and babies.

There is no vaccine for the common cold, which is caused by rhinoviruses.

But with all respiratory viruses, people can make sure to dress appropriately with hats and gloves in the cold. 

Additionally, a mask can help. In other countries, a mask can show someone is sick, and they’re protecting others nearby, Rajendram said. But if people are sick, they should try to stay home to avoid infecting others.

It's also a good idea to avoid touching your face. People can spread bacteria or virus from their mouth or nose and then shake someone’s hand or touch a surface, leaving a pathogen behind. If you have to cough or sneeze, avoid using your hand and instead cough into the crook of your arm or a tissue. And make sure to wash your hands frequently with soap and water.