What is the flu?

In order to discuss why we have flu season, we must first understand what the flu is. Influenza, also called the flu, is a viral disease of the respiratory tract. A virus is a microscopic infectious agent that attacks your body's cells and makes you sick. The flu is often confused with another virus, the common cold, because of the similarity in symptoms, which can include a cough, sore throat, and stuffy nose. However, flu symptoms also include fever, cold sweats, body aches, headaches, exhaustion, and even some gastrointestinal symptoms like vomiting and diarrhea.
There's a chill in the air, and you all know what that means—it's cold and flu season, when everyone you know seems to be suddenly sneezing, sniffling, or worse. It's almost as if those pesky cold and flu germs are swirling with the first onslaught of winter weather.
Yet, germs are present year-round—just think of your last summer cold. So why do people get colds, flu, and now Covid-19 more often when it's cold outside?
In what scientists are calling a scientific breakthrough, researchers behind a new study may have found a biological reason why we have more respiratory illnesses in the winter. Cold air itself has been shown to impair the immune response occurring in the nose.
"This is the first time we have a biological, molecular explanation for one factor in our innate immune response that appears to be limited by colder temperatures," said rhinologist Dr. Zara Patel, professor of otolaryngology and head and neck surgery at Stanford. University School of Medicine in California. She was not involved in the new study.
In fact, lowering the temperature inside the nose by just 9 degrees Fahrenheit (5 degrees Celsius) kills nearly 50% of the billions of virus- and bacteria-fighting cells in the nostrils, according to a study published Tuesday in The Journal of Allergy. And clinical immunology.
"Cold air is associated with increased viral infection because you've essentially lost half of your immunity with just that small drop in temperature," said rhinologist Dr. Benjamin Bleier, director of otolaryngology translational research at Massachusetts Eye and Ear and associate professor at Harvard. Boston College of Medicine.
"It's important to note that these are in vitro studies, meaning that although human tissue is used in the lab to study this immune response, this is not a study done inside someone's actual nose," Patel said in an email. "Often the results of in vitro studies are confirmed in vivo, but not always."
Hornet's nest
To understand why this is happening, Bleier and his team and co-author Mansoor Amiji, who chairs the department of pharmaceutical sciences at Northeastern University in Boston, began a scientific follow-up.
A respiratory virus or bacteria invades the nose, the main point of entry into the body. The team found that the front of the nose immediately detects the germ, long before the back of the nose becomes aware of the intruder.
At this point, the cells lining the nose immediately begin making billions of simple copies of themselves called extracellular vesicles, or EVs.
"EVs can't divide like cells, but are like little mini versions of cells specifically designed to kill these viruses," Bleier said. "EVs act as decoys, so when you inhale the virus, the virus sticks to these decoys instead of the cells."
These “Mini Me” are then pushed by the cells into nasal mucus (yes, snot) where they stop attacking germs before they reach their destination and reproduce.
"This is one of, if not the only, part of the immune system that leaves your body to go fight bacteria and viruses before they actually enter your body," Bleier said.
Once created and dispersed into nasal secretions, the billions of EVs begin swarming with harmful bacteria, Bleier said.
“It's like kicking a hornet's nest, what happens? You can see a few hornets flying around, but if you kick it, they all fly out of the nest and attack before the animal gets into the nest itself,” he said. "This is how the body clears these inhaled viruses so, they can never enter the cell."
The great increase in immune power
When infected, the nose increases the production of extracellular vesicles by 160%, the study found. There were other differences, too: EVs had many more receptors on their surface than native cells, increasing the ability of the billions of extracellular vesicles in the nose to stop the virus.
"Think of the receptors as little arms that stick out and try to catch virus particles when you inhale them," Bleier said. "And we found that each vesicle has up to 20 times more receptors on the surface, making them super sticky."
Cells in the body also contain a viral killer called micro RNA that attacks invading bacteria. Yet EVs in the nose contained 13 times more micro RNA sequences than normal cells, the study found.
So the nose comes into battle armed with some extra superpowers. But what happens to these benefits when the cold weather hits?
To find out, Bleier and his team exposed four study participants to temperatures of 40 degrees Fahrenheit (4.4 degrees Celsius) for 15 minutes and then measured the conditions inside their nasal passages.
“We found that when you're exposed to cold air, the temperature in your nose can drop as much as 9 degrees Fahrenheit. And that's enough to basically knock out all three of the immune benefits that the nose has," Bleier said.
In fact, that little bit of cold on the tip of the nose was enough to knock out nearly 42% of the extracellular vesicles, Bleier said.
"Also, you have almost half the amount of these killer micro RNAs inside each vesicle, and you can have up to a 70% decrease in the number of receptors on each vesicle, so they're much less sticky," he said.
What does it do to your ability to fight off colds, flu, and Covid-19? It cuts your immune system's ability to fight respiratory infections in half, Bleier said.
You don't have to wear a sock on your nose
it turns out, the pandemic has given us exactly what we need to help beat the cold air and keep our immunity high, Bleier said.
"Not only does masked rhinologist Dr. Benjamin Bleierotect you from directly inhaling viruses, but it's like wearing a sweater over your nose," he said.
Patel agreed: “The warmer you can keep the intranasal environment, the better this innate immune defense mechanism will work. Maybe another reason to wear masks!”
In the future, Bleier expects to see the development of topical nasal medications that build on this scientific discovery. These new drugs "basically trick the nose into thinking it's just a virus," he said.
"By having that exposure, you're going to have all those other hornets flying around in your mucosa to protect you," he added.