Updated on December 10, 2024.
Flu is a respiratory illness caused by the influenza virus. Symptoms can range from mild to severe, but it’s important to recognize that the sickness is different than the common cold, COVID-19, or respiratory syncytial virus (RSV).
These four illnesses are caused by different viruses. They may share some similar symptoms, including fatigue, sore throat, runny nose, and a cough. Flu and COVID also both share symptoms like body aches, fever, and headaches. But there are some differences, too:
- A cold will tend to be much milder, and usually doesn’t cause symptoms like a fever or body aches. It also tends to run its course in a few days.
- Symptoms of COVID can include the loss of smell or taste.
- RSV often has wheezing, while the others do not. Wheezing sounds like a high-pitched whistle when you breathe out.
Flu often develops more quickly after a person is exposed–between one and four days after, whereas COVID can take up to two weeks to show symptoms after exposure. And while there are no useful tests for identifying a cold, there are reliable tests for both COVID, RSV, and the flu. If you’re unsure which illness you have, it’s best to get tested.
For some people, the flu can be dangerous, even life-threatening. The flu and related flu complications claimed an estimated 21,000 lives during the 2022-2023 season, according to the Centers for Disease Control and Prevention (CDC). Between 2010 and 2019, the death toll reached anywhere from 12,000 to 51,000 a year.
RSV can cause serious illness, too. Up to 160,000 people ages 65 and over may be hospitalized with RSV each year in the U.S., and up to 10,000 of these people may die from causes related to RSV.
Anyone can catch the flu, but some populations are more likely to have adverse reactions. Find out who is at the highest risk and which symptoms may warrant emergency care.
Who is most at risk for flu complications?
Certain people are more at risk of experiencing dangerous side effects from the flu than others. These include the following groups:
Children: Because their immune systems are still developing, children under the age of 5—and especially those under age 2—are vulnerable to dangerous influenza complications. Between 2010 and 2020, between 6,000 and 27,000 children younger than 5 were hospitalized for issues related to the flu each year, according to the CDC.
Pregnant people: Pregnancy alters the immune system, heart, and lungs, which can mean that getting the flu during pregnancy may increase the risk of severe symptoms and flu complications like pneumonia.
“Your physiology, blood pressure, blood flow, and heart rate are all different during pregnancy, because the fetus is taking up a lot of your blood flow and nutrients,” says Matthew Tincher, MD, an emergency medicine specialist with TriStar Horizon Medical Center in Dickson, Tennessee.
This increased risk for flu complications lasts up to two weeks after giving birth. The flu can also cause problems for a developing fetus, including premature delivery and fetal brain, spine, and spinal cord defects. Getting a flu shot during pregnancy can provide some protection after the birth of the infant, before they are old enough to be immunized.
Adults 65 and older: As we age, our immune systems become weaker. The CDC estimates between 70 percent and 85 percent of flu-related deaths occur in adults over 65. Older adults make up 50 percent to 70 percent of seasonal flu hospitalizations.
“Their immune systems are not fully active, so it’s easier for them to pick up the virus,” Dr. Tincher explains.
People with medical conditions: The flu weakens your body and can make existing health conditions worse. For this reason, people with conditions like asthma and diabetes may have a harder time managing symptoms while they're ill. One example: Infections like the flu make it harder for people with diabetes to control blood sugar levels.
Influenza can also increase your heart attack risk, according to the CDC. Past research suggests that adults may be six times more likely to have a heart attack within a week of a flu diagnosis than one year before or after the sickness. About half of adults hospitalized with flu during recent flu seasons had cardiovascular disease.
People with other health conditions are also at greater risk for flu-related complications. These include:
- Chronic lung diseases, like chronic obstructive pulmonary disease and cystic fibrosis
- Liver and kidney disorders
- Neurological conditions, like epilepsy and cerebral palsy
- Blood disorders, like sickle cell disease
- Diseases or medications that weaken your immune system, like cancer, human immunodeficiency virus (HIV), or steroids
- A body mass index, or BMI, of 40 or greater
If you experience flu-like symptoms and have any of these medical conditions, talk to your healthcare provider (HCP) about getting the proper treatment.
When is emergency care necessary?
Flu symptoms typically come on suddenly within one to four days of exposure to the virus. The illness can last anywhere from a few days to two weeks. Most cases of influenza are mild and can be managed at home, without the help of a health professional. Common symptoms include a cough, chills, headaches, fatigue, muscle aches, and sore throat. Not everyone with the flu will have a fever, but most do. If your symptoms are more severe, reach out to an HCP.
"Not everybody with the flu needs to come to the emergency department," Tincher says. "The vast majority of healthy adults and children can get treatment for the flu in their doctor’s office." If you can’t get an appointment with an HCP, visit your local urgent care facility.
In certain cases, some people can develop moderate or serious complications from the flu. These can include pneumonia, sepsis (this is an extreme response to an infection by your immune system; it can damage organs), sinus or ear infections, and bronchitis. Both pneumonia and sepsis can be life-threatening and need prompt medical treatment.
Adults who experience these symptoms with flu should contact an HCP or visit urgent care immediately:
- Difficulty breathing or shortness of breath
- Severe or persistent vomiting
- Chest or belly pain or pressure
- Sudden dizziness
- Confusion
- Dehydration
- Symptoms that get better, then return with fever and a worsened cough
In infants and children, watch for trouble breathing, a fever accompanied by a rash, lack of tears when crying, or skin that is bluish in color. “Children with the flu may show a decreased level of activity, a fever, rapid breathing, or coughing,” says Tincher. If your young one has any of these symptoms, it could be a sign of a flu complication or severe illness, he adds.
“A healthy child should be active, smiling, and playful,” Tincher says. If your child isn't interactive or is acting fussy and you think it could be due to the flu, he recommends heading to the emergency department (ED) at your nearest hospital.
What’s the best way to prevent the flu?
The CDC recommends the flu vaccine for anyone six months and older. You can get the vaccine either as an injectable shot or a nasal spray, depending on your age and health. Children under the age of eight who have not been vaccinated before should receive two doses of the vaccine at least 28 days apart. If you're feeling ill or have an allergy to the vaccine or any of its ingredients, speak with your HCP before getting immunized. If necessary, your HCP can reschedule your vaccination appointment once you are feeling better and provide you with other tips for staying virus-free.
Research suggests that vaccination typically reduces your flu risk by 40 percent to 60 percent, though the effectiveness of the shot varies each year. Even if the shot doesn’t prevent the flu, it can reduce severity of symptoms and protect against dangerous flu complications.
Flu outbreaks typically peak between December and February, but cases can occur as early as October and as late as May. The CDC recommends getting the flu vaccine before the end of October, but vaccinations given at any time during the flu season can help safeguard against sickness.
What are special versions of the flu vaccine?
The nasal spray version of the flu vaccine is not recommended for children under two years of age, adults over 49 years, people who have a weakened immune systems, or anyone between the ages of two to 17 who take aspirin-containing medications. Pregnant people should not receive the nasal spray. Despite some persistent misconceptions about the flu shot, the standard shot is safe for pregnant people.
A high-dose vaccine is also available for adults older than 65, and research suggests it may be more effective in preventing flu in older adults than the standard flu shot.
Vaccines protect more than the people who are vaccinated. They also help prevent transmission to those who aren't. When vaccinated people help prevent transmission to those who can’t be vaccinated, it’s known as herd immunity. This is especially important for infants and people with conditions that restrict their ability to get the flu shot.
For herd immunity to be effective, a certain percentage of the community must get vaccinated. The more people who get the flu shot, the greater protection we have against the flu.
It’s also important to regularly check to see if you have received the latest updated COVID vaccine and make sure you’re up to date with other CDC-recommended boosters and vaccines.
How do you steer clear of the flu?
The flu shot is just the first step in preventing the flu. Tincher also recommends staying away from sick people when you can. Other infection-reducing steps include:
- Washing your hands regularly with soap and water for at least 20 seconds
- Avoiding touching your nose, eyes, and mouth
- Wearing a face mask, especially if you have a respiratory infection. Masks reduce the spread of viruses to other people. Consider wearing a mask if you are around other people who may transmit an infection, such as large groups of people or during airplane travel.
- Routinely disinfecting objects and surfaces in your home, office, and school
- Managing stress with techniques like physical activity, medication, or calling a friend
- Getting enough sleep (about seven to nine hours a night)
- Eating a healthy, well-balanced diet
These steps will also help protect you from other viruses, including those that cause COVID, the common cold, and RSV. Help keep others flu-free by staying home for at least 24 hours after your fever subsides and covering your mouth when you cough or sneeze.
