What You Need To Know About the Cold and Flu Season

Though winter may have many of us hunkering down amid cold snaps and snowstorms, it's also a time that ushers in some unwelcome visitors: colds and flu. Adults average two to three colds a year, and each year 5%-20% of the U.S. population catches the flu, according to the Centers for Disease Control and Prevention. Knowing the facts about colds, flu, and flu shots, such as knowing who's most at risk, the difference between colds and the flu, how effective flu shots are, and the latest breakthroughs in cold and flu season research, can help guide you through the nastiest cold and flu season.

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One important thing you need to know is, influenza is caused by a virus — not bacteria — which means those antibiotics you ask your doctor for are useless when it comes to treatment. The CDC warns against taking antibiotics unnecessarily because it can help bacteria become more resistant to treatment when it’s needed. 

As many as 200 different viruses could be causing your sniffles, but the most common culprit is the rhinovirus. It accounts for up to half of all colds and can survive for as long as two days outside of the body — especially on hard surfaces such as doorknobs and faucet handles. (So, wash your hands.)

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The CDC classifies flu viruses as A, B, C, and D. Types A and B are seasonal flu, and influenza A is broken down further into subtypes and strains. For example, current subtypes include H1N1 and H3N2. Types B and C typically aren't as severe as type A. (Type D, a relatively new discovery, affects pigs and cattle.)

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Flu viruses undergo continual genetic changes that can accumulate over time, which is why the flu is a continual risk even after we develop antibodies to fight a certain kind. But flu viruses can also undergo sudden shifts that cause major pandemics, such as when H1N1 — otherwise known as swine flu — spread quickly in 2009.

You can find just about every type of influenza A in birds. For that reason, scientists think most flu is bird flu at its root. The bird flu we hear about most often, also known as H5N1, typically is transmitted from bird to bird, but can infect humans in rare cases after close contact with infected birds. Additionally, research suggests that cold viruses also came from birds — along with camels.

Though winter seems like the obvious time of year for flu, the season can stretch from October to May. Though you can get the flu during the warmer months, you're much more likely to succumb to a virus with some flu-like symptoms instead of influenza.

Until recently, scientists didn't know why colds and flu peak during cold weather — just that they do. But in what's being called a scientific breakthrough, researchers have found the biological reason we get sick more often in winter. Cold air damages the immune response that occurs in the nose, according to a study published in the Journal of Allergy and Clinical Immunology in December 2022. The drop in temperatures in winter can kill nearly 50% of the billions of disease-fighting cells in the nostrils, leading to a severely weakened frontline immune defense. 

An estimated 28,000 people died from the flu during the 2023-2024 season (of about 40 million instances of the disease), according to CDC data. Several factors make precise mortality estimates difficult, including the fact that flu may trigger another infection or aggravate an existing condition. In such cases, flu isn't listed as a cause of death despite its role.

A flu pandemic in 1918 and 1919 killed 20 million to 50 million people worldwide and sickened more than a fourth of the U.S. population at the time. But the likelihood of flu killing people on that kind of scale again is low because of advances in monitoring and drugs that can treat secondary infections.

For the most part, our bodies are able to effectively fight off a cold. But mutations of the common cold virus have proven occasionally deadly. For instance, in 2007, at least 10 patients died after an adenovirus, a bug that causes colds, mutated into a more severe respiratory infection.

Though most healthy adults who get the flu experience mild illness, some groups are at higher risk for complications or death. They include young children, especially under age 2; adults 65 or older; pregnant and postpartum women; residents of nursing homes; and Native Americans.

Kids are crammed into close quarters at school or daycare, and they’re also unlikely to wash their hands long enough to prevent the spread of germs. Combine all that with their still-tender immune systems, and you have the recipe for a long, sneezy winter.

Certain conditions also put you at a higher risk of complications arising from the flu. Those include respiratory conditions such as asthma or emphysema; diabetes; stroke; heart disease; cancer; kidney problems; HIV or AIDS; and obesity.

Even if you have only a cold, that doesn’t mean it will stay completely harmless. Colds can trigger secondary, more serious infections such as pneumonia, or can make you more susceptible to secondary infections, such as strep throat, the Mayo Clinic says. They can also trigger asthma attacks, ear infections, and sinusitis.

People often mistake a common cold for the flu because some common-cold symptoms such as stuffy nose, sneezing, and sore throat can accompany the flu. Severe body aches, weakness, exhaustion, high fever, and intense chest discomfort are all signs you probably have the flu instead of a cold. Another clue? Colds usually start gradually, while flu can be quite abrupt, according to the CDC.

It might make you feel better to call that tummy bug "stomach flu," but it's probably gastroenteritis, not influenza, according to the Mayo Clinic. The hallmarks of gastroenteritis are intestinal-based symptoms including vomiting and diarrhea, while the real flu centers on your respiratory system.

No matter what grandma says, you won’t come down with a cold just because you went outside without your coat on. You catch a cold only from another person who has it, whether through direct contact or touching a germy surface, according to WebMD. Though we now know that cold air weakens the immune response in your nostrils, you still won't contract it without coming into contact with the virus.

During the 2023-2024 flu season, the flu vaccine was about 59% to 67% effective, the CDC estimates. Year to year, numbers can vary widely because officials have to guess which kinds of flu viruses will circulate most widely, then formulate a vaccine to protect against three to four viruses. In 2014-15, flu shots were only about 19% effective because there was a mismatch between prevalent flu strains and what was in the vaccine.

Though experts admit a flu shot doesn't offer 100% protection, the CDC recommends one for everyone ages 6 months and older. The flu vaccine prevented an estimated 1.8 million illnesses and 1,000 deaths in 2021-2022, according to CDC data. And even if you do still get the flu, it's likely be much milder than it would have been without a flu shot, researchers have found.

Alas, your doctor won’t be offering you a cold vaccine anytime soon, though tests blocking a human protein that colds need to replicate have shown some promise. For now, we’re still stuck following doctors’ timeworn advice: chiefly, to rest and stay hydrated to avoid prolonging the misery.

Though most officials say anytime is a good one for a flu shot, older people in particular may want to hold off when flu-shot signs start popping up in September. That's because the shot may not be quite as effective for quite as long when you're over 65. Experts tell NPR that a good window is between Halloween and Thanksgiving for optimal protection when flu season peaks, typically in late winter. 

The flu viruses in vaccines are inactive, which means they can't infect you, the CDC says. The most likely side effects are tenderness and minor swelling where the shot was given. It takes about two weeks for the shot to cause antibodies to develop in the body, so it's still possible to get sick after getting the shot.

Most flu vaccines are formulated with a small amount of egg proteins, but the CDC says the chance of a severe reaction in patients with egg allergies is very low. Those who experience hives after eating eggs can get the vaccine anywhere it's offered, but patients with more severe allergies should stick to an inpatient or outpatient medical setting such as a clinic or physician's office to be on the safe side. There's also an egg-free vaccine called Flublok.

Despite fears to the contrary, the flu shot is safe for women who are pregnant or breastfeeding. Women can get vaccinated during any trimester — even the first — and the antibodies their bodies make after getting the flu shot can help protect babies for the first six months after birth when they are too young for their own shots. (The nasal spray flu vaccine FluMist, however, isn't recommended for pregnant women.)

Flu shots are no substitute for common-sense measures you can take to cut down on your risk of getting sick, including limiting contact with others who have the flu, frequent and prolonged handwashing, using alcohol-based hand sanitizers, refraining from touching your face, and getting plenty of sleep and exercise.

When you get the flu, you're first contagious for about a day before symptoms start and about five to seven days after symptoms show, according to Healthline. The news is worse for colds: You can be contagious two days before having symptoms, and for two weeks after being exposed to the virus.

Prescription antiviral medications such as Tamiflu typically shorten only the duration of flu symptoms, and not by much, according to the Cleveland Clinic. Researchers have found the medicine to shave a day of symptoms off a weeklong illness. The drug is also costly and has its own potential side effects. Still, it might be worthwhile for patients at high risk for flu-related complications.

Once you have the flu, treatment is much the same as it is for other common viruses, assuming you forgo antivirals such as Tamiflu: plenty of rest and fluids. Over-the-counter pain relievers including ibuprofen can help relieve body aches and fever temporarily. Using a cool-mist humidifier, breathing in steam, and using saline nose drops can help alleviate congestion.

Cold and flu germs don't last long outside the body, so scrubbing every countertop and doorknob to stay healthy might not be worth your while. Though there's a chance some germs could last up to a day or two on hard surfaces, direct contact with an infected person is the much greater risk. That makes hand-washing much more important, according to the Mayo Clinic.

Though some consumers swear by natural products such as zinc, vitamin C, echinacea, or other vitamins and herbs during cold and flu season, experts say there's no concrete evidence that they help prevent colds or flu. Zinc may help cut down on the duration of colds, but it can also cause side effects or allergic reactions. Consult a doctor before using zinc or any other natural remedy.

From October through May, the CDC monitors flu activity nationwide based on lab tests, physician visits, hospitalizations, and more. A flu activity map can give a snapshot of flu activity in your state, whether minimal or widespread.