Why is it easier to catch a cold, flu or covid in the winter?

When the bitter winds were blowing and the heat was emitting, my grandmother urged me to come inside. “You’ll freeze to death there,” he said.

Freezing to death can be certain in cold temperatures. But doctors and other health experts have long emphasized it to be- the cold will not give you the cold. Winter is still undoubtedly cold and flu season. It’s also a time when COVID-19 is spreading more.

But if the cold doesn’t matter, why the spread of so many respiratory viruses in peak season?

“We were looking at this problem 13 years ago,” says Linsey Marr, a civil and environmental engineer at Virginia Tech in Blacksburg who studies airborne viruses. “The deeper we go, the more we know we don’t know” [and] it is more like to be dry.

They are not alone. “In the winter season that people hesitated for a very long time; thousands of years, to be honest,” says Jeffrey Shaman, an infectious disease researcher who directs the Climate and Health Program at Columbia University’s Mailman School of Public Health.

It is certain that some shorter winter days make people more susceptible to infection, he says. Less sun means people make less vitamin D, which is required for some immune responses. But that’s just a puzzle.

Experts are also looking at what other factors may play a role in making the winter sicker.

The disease spreads more within.

My grandmother urged Will to be able to come in from the cold rather than increase the risk of becoming seriously ill.

Colds, influenza and respiratory syncytial virus, or ARV, are all diseases that are more prevalent at certain times of the year when people spend more time indoors. It includes winter in temperate climates, where there are distinct seasons, and rain in tropical zones. COVID-19 is also spreading more internally than externally (SN: 6/18/20).

Diseases are caused by viruses, which are transmitted primarily through breathing in small droplets known as aerosols. It is a change in thinking. Until recently, many scientists thought that such viruses were spread mainly by touching contaminated surfaces (SN: 12/16/21).

“When you’re outside, you’re in the last well-ventilated area,” says David Fisman, an epidemiologist at the University of Toronto’s Dalla Lana School of Public Health. Viruses exhaled outside are quickly diluted in clean air.

But inside, aerosols and the viruses they contain can build up. “When you’re in a poorly ventilated space, you’re breathing in the same air that other people have breathed in,” he says.

Since viruses come with that exhaled breath, “it makes a lot of sense that proximity to individuals who would be contagious would facilitate transmission,” Shaman says.

But there is more to the story, says Benjamin Bleier, a specialist for sinus and nasal disorders at the Harvard School of Public Health.

“In today’s society, we are home all year round,” he said.

To drive the model for the time of year we see, something else is going to happen too much to make people vulnerable to infection and increase the amount of virus circulating, he says.

The drier air can give the virus a bit of a boost.

Some viruses thrive in the winter. But the reason why is not so much about the temperature, but about the humidity.

“There are some viruses that like it hot and humid, and some viruses that like it dry and cold,” says Donald Milton, an aerobiologist at the University of Maryland’s College Park School. For example, rhinovirus—one of the many types of viruses that cause colds—survives better when it’s moist. The causes of rhinovirus infection typically peak in the early fall.

Marr and other researchers have found that viruses that circulate in the winter, including influenza viruses and SARS-CoV-2 — the coronavirus that causes COVID-19 — survive best when the relative humidity in the air drops below about 40 percent.

Viruses don’t usually fly around naked, Marr says. They are deposited in drops of fluids, such as saliva. Those drops also have layers of mucus, proteins, salts and other substances. Those other parts can determine if the virus is dried.

Where the humidity is higher, the drops dry more slowly. Such drought kills viruses such as influenza A and SARS-CoV-2, Marr and colleagues reported July 27 in a bioRxiv.org preprint. During slow drying, salt and other substances that can harm viruses become denser, although researchers still do not fully understand what happens on a molecular scale to activate viruses.

But flash drying in the air keeps the virus dry. “If the air is very dry, the water evaporates very quickly. Everything is dry, and almost like concrete in place,” said Marr.

Drier, smaller aerosols are also softer and hang in the air longer, increasing the chance that someone will inhale them, Fisman says.

What’s more, dry air can break down some people’s defenses against viruses. Studies in animals suggest that dry air can trigger the death of certain cells lining the airways. That could leave cracks where viruses can invade.

Mucus in the airways can trap viruses and help protect against infection. But breathing cold, dry air can also slow down the system that usually moves the slimy body. That can give the virus time to break out of the mucus traps and invade cells, Fisman says.

Let the cold hurt the strength to fight.

Being cold may not give you a cold, but it may make you more apt to catch one.

Basically, the immune system has a trick to ward off viruses, Bleier and colleagues recently discovered. Cells in the nose and elsewhere in the body have superficial patches that can detect viruses. When one of the proteins sees a virus coming from the cell, it signals the cell to release small vesicles called extracellular vesicles.

The bubbles work as a different technique, a bit like a straw being thrown from a military force looking for a hot missile, Bleier says. Viruses can infect cells behind vesicles.

If the virus docks with one of the bubbles, there is a surprise in it: Inside the vesicles, the virus will be killed by RNA, which are called microRNAs. One of those microRNAs that miR-17 It could kill the two types of rhinoviruses that cause the common cold, the team reported on December 6. Journal of Allergy and Journal of Immunology.

The researchers measured bubbles released from human nasal cells in lab dishes at 37° Celsius, our typical body temperature. The scientists then lowered the thermostat to 32° C. Cells produced about 42 percent fewer vesicles at the cooler temperature, the team found. Moreover, those vesicles carried fewer weapons. The vesicles can store about 24 percent more microRNA at body temperature than when it is colder.

Three tips to strengthen our immune system.

I asked the experts what people can do to protect themselves from the virus in winter. Some have said that using a humidifier can help raise humidity levels enough to slow the drying of virus-friendly droplets, killing the virus.

“Any increase in humidity should be beneficial,” said Flamini. “You get a lot of bang for your buck if you go from dry to dry.”

But Milton doesn’t think it’s a good idea to pump a lot of water into the house when it’s cold outside. That moisture will find all the cold in your house and condense there, creating, he says, mold and rot.

But advocates turning to kitchen and bathroom exhaust fans to increase ventilation and use HEPA filters or Corsi-Rosenthal boxes to filter unwanted viruses from the air (SN: 7/25/22).

Bleier suggests masked. Not only can larvae filter out viruses, but “our work suggests these individuals have a secondary mechanism of action.” “Keep warm and keep” [moist] air in front of our noses, which could help the immune system.