The flu basics
By Jenilee Matz, MPH Jun 13, 2021 • 6 min
The flu is a respiratory illness that affects millions of people each year.
It's caused by a virus that spreads easily from person to person. Learn how to protect yourself from the flu virus and what to do if you become ill.
What is the flu?
Influenza (the flu) is a viral respiratory infection of the nose, throat and sometimes lungs. Illnesses can range from mild to severe. In some cases, complications from the flu can lead to hospitalizations and even death. Possible complications include bacterial pneumonia and ear and sinus infections. The flu can also make chronic health conditions worse, including diabetes, asthma and congestive heart failure.
Types of influenza
There are four types of flu viruses. Human influenza types A and B viruses are responsible for most seasonal flu viruses in the U.S., while influenza type C is believed only to cause mild illnesses and not outbreaks. Influenza type D mainly affects cattle and isn't known to cause infections in humans. Each type of flu virus is further broken down into different strains, and strains can change over time.
Influenza A viruses are also found in many different animals, including birds and pigs. While it's unusual for people to get influenza infections directly from animals, sporadic human infections and outbreaks caused by certain avian influenza A viruses (or bird flu) and swine flu viruses, including H1N1 flu, have occurred. One of the deadliest influenza outbreaks in history, the 1918 flu pandemic (known as the Spanish flu) was believed to be caused by an H1N1 virus with genes of avian origin.
How do you get the flu?
The flu is thought to be spread through tiny droplets that come out of an infected person's mouth during sneezing, coughing or talking. People nearby can inhale the infected droplets and become sick. You can also catch the flu if the infected droplets get on your hands and you touch your eyes, nose or mouth.
When is flu season?
Flu season occurs in fall and winter in the U.S., generally between October and May. While the flu virus is always going around, flu activity tends to peak between December and February. The Centers for Disease Control and Prevention (CDC) updates a flu map weekly during flu season, so you can see flu activity in your state.
The CDC recommends that everyone age 6 months and older receive a flu shot each year. Flu vaccination can prevent flu illnesses, healthcare provider visits and hospitalizations, and it can even be lifesaving in children. (Schedule an appointment for a flu shot.)
Most people who receive the flu shot don't have any problems. But as with any medication or vaccine, including the flu shot, there is a chance of reactions. Flu shot side effects are generally minor and go away on their own within a few days. The most common side effects include soreness, redness and tenderness at the site on your skin where you received the shot. Some people may have a low-grade fever, headache or muscle aches. While severe reactions are uncommon, you should let your healthcare provider know if you have a history of allergy or severe reaction to the flu vaccine or any part of the flu vaccine. Know that you cannot get the flu from the flu shot.
What are the symptoms of the flu?
Flu symptoms usually come on suddenly. Flu symptoms in adults include:
- Fever above 100°F
- Body aches and pains
- Fatigue and weakness
- Sore throat
Children with the flu may have the same symptoms as adults, but young children may have different symptoms. Additional flu symptoms in kids can include:
- High fever
- Febrile seizures (convulsions that happen during a fever)
- Gastrointestinal symptoms, including nausea, vomiting, diarrhea and loss of appetite
How long does the flu last?
The flu tends to last about one week, but some signs of the flu, including weakness and fatigue, may linger for several weeks. A fever due to the flu lasts about two to five days.
How long is the flu contagious?
The flu is most contagious during the first three to four days of symptoms. However, it's possible to infect others one day before you feel sick, and up to five to seven days after symptoms begin. People with weakened immune systems and young children may be able to spread the virus to others for a longer time.
When is the flu contagious?
The flu incubation period, or time between exposure to the virus and when symptoms start, is typically two days, but it can range from one to four days.
How is flu testing done?
Healthcare providers often diagnose the flu based on symptoms alone. In many cases, there's no need to test for the flu since the results won't change the treatment. But sometimes healthcare providers do lab testing for the flu. This involves your provider swabbing the back of your throat or inside of your nose. They'll run a rapid test on the specimen in their office or send the specimen out to a lab to be analyzed.
How to treat the flu
There is no cure for the flu. In fact, antibiotics aren't effective treatments for flu. This is because the flu is a virus, and antibiotics only work against bacterial infections. Most people recover from the flu without needing to see their healthcare provider. Tried and true home remedies for the flu include getting plenty of rest and drinking enough fluids. Using over-the-counter flu medicine to address congestion, cough and fever can provide temporary relief of your symptoms.
In some cases, antiviral medications are prescribed for flu treatment. Antiviral flu medicines include oseltamivir (Tamiflu), zanamivir (Relenza), peramivir (Rapivab) and baloxavir (Xofluza). When treatment is started within two days of becoming sick with flu symptoms, these medications can reduce the severity of the illness and duration by about one day, and may also lower the risk of flu complications, such as ear infections, pneumonia and hospitalizations. Antivirals are mostly prescribed to those who are severely ill or at high risk of serious flu-related complications. Still, even if you aren't severely sick or not at high risk, your healthcare provider may prescribe an antiviral if you've been sick with the flu for fewer than 48 hours. Antiviral medications work best when started in this time frame. Discuss with your healthcare provider if an antiviral is right for you. Sometimes, healthcare providers may prescribe antivirals for prophylaxis (prevention) of the flu.
Who is at risk for flu complications?
Most people recover from the flu without problems. However, certain groups of people are at high risk of developing complications from the flu. If you have symptoms of the flu while pregnant, call your healthcare provider. Children 5 years and younger (especially those 2 years and younger) and people 65 years and older should also contact their provider at the first sign of early flu symptoms. The risk of complications is also high in people with chronic medical conditions, like diabetes and asthma, and those with weakened immune systems due to human immunodeficiency virus (HIV) or receiving a transplant, for instance.
Sometimes, healthy people can get complications from the flu. It's a good idea to call your healthcare provider if your symptoms aren't getting better or are getting worse. Other worrisome symptoms in adults include:
- Shortness of breath or trouble breathing
- Chest or stomach pain or pressure
- Dizziness when you stand up
- Severe vomiting or not being able to keep fluids down
Call your child's healthcare provider if they develop these symptoms:
- Fever with a rash
- Blue- or purple-colored skin
- No tears when crying (in infants)
- Extreme irritability (infants may not want to be held)
- Not waking up easily
Can dogs get the flu?
Dogs can catch canine influenza or dog flu from other dogs. If your dog comes down with the flu, know that they likely cannot pass it to you. Flu viruses are always changing, but to date, there haven't been any known cases of dog flu in humans. The reverse is also likely to be true. If you have the flu, you probably don't have to worry about your dog catching it from you.
The difference between cold and flu
It can be challenging to tell the difference between a cold and the flu since both illnesses cause similar symptoms. In short, the flu often presents with a fever, body aches, chills and exhaustion, while a cold normally doesn't. Colds commonly cause a sore throat, sneezing and stuffy or runny nose, while the flu only does sometimes. At-home treatments for colds and flu are often the same and include plenty of bed rest and fluids.
What is the stomach flu?
The terms "stomach flu" and "24-hour flu" are misnomers. The flu is a respiratory virus and doesn't usually cause gastrointestinal symptoms, like vomiting, diarrhea and abdominal pain or cramping, though it can in children. Viral gastroenteritis or stomach bugs are intestinal infections. Likewise, people often inquire about food poisoning vs. flu. The two tend to cause very different symptoms. Foodborne illness often causes gastrointestinal discomforts, while the flu causes respiratory symptoms.
What is H-flu?
H-flu or Haemophilus influenza isn't the same as the influenza. Rather, it's a kind of bacteria that causes many types of infections, such as ear infections and pneumonia.
In summary, the best way to prevent the flu is to get the flu vaccine. If you're already under the weather and wondering, "do I have the flu?"—take heart in knowing that most people get better on their own without problems. Rest, drink plenty of fluids and consider using over-the-counter medicines to ease symptoms. If you aren't getting better, are getting worse or are at risk for complications, call your healthcare provider.
Clinically reviewed and updated June 2021.