What is the flu?

It’s flu season. With the coronavirus pandemic front and center, worrying about flu season almost seems like an afterthought. Though COVID-19 and influenza are both respiratory illnesses with many of the same symptoms, they are caused by two completely different viruses.

So what is the flu? An often-misused term, the flu is a highly contagious respiratory illness caused by one of three virus types: influenza A, B or C. (Influenza type D infects pigs and cattle, but not humans.) According to the Center for Disease Control (CDC), A and B are the most common virus types in humans. Vaccines are developed each year for these two types to help prevent the spread of the seasonal flu. Influenza type A is the virus type associated with global pandemics. Influenza type B is generally less severe but still causes outbreaks like we saw this past flu season (2019–2020). Type C causes very mild symptoms.

The flu “season” in the United States generally coincides with winter months, although the viruses are detected year-round. Flu activity picks up in late fall, usually peaks between December and February, and sometimes sticks around until late spring.

Flu symptoms

Symptoms of the flu are similar but more severe than those of the common cold. Notably, the flu appears suddenly, rather than gradually, with symptoms such as:

  • Fever and/or chills
  • Fatigue
  • Body aches
  • Cough
  • Sore throat
  • Runny or stuffy nose
  • Headaches
  • Vomiting and diarrhea (sometimes)

Symptoms show up 1 to 4 days after exposure to the virus. This means it’s possible to pass the flu to someone else before realizing you’re infected, in addition to when symptoms are apparent. People showing no signs of the flu (asymptomatic carriers) can also pass the virus to others. People are most contagious in the first 3-4 days after the virus enters the body. However, they can continue to spread the virus for up to a week after catching it.

The flu virus spreads via the air through tiny droplets expelled when people cough, sneeze, or even talk. People infected with the flu can pass it to those who are up to 6 feet away. It’s also possible to catch the flu by touching a surface or object where the virus has landed and then touching your mouth, nose or eyes.

Flu prevention

Ben Franklin said, “An ounce of prevention is worth a pound of cure,” and that’s certainly true for the flu. While a vaccine is available each year for the seasonal flu, common-sense hygiene can play a large part in preventing infection and lessening the severity or duration of symptoms. Preventative tactics include:

  • Get the annual flu vaccine
  • Wash your hands with soap and water for at least 20 seconds before eating or touching your face
  • Reduce contact with people who are sick
  • Quit smoking
  • Contact your doctor promptly if flu symptoms appear

The earlier the diagnosis, the better. Medicines like Tamiflu, Xofluza, and other antivirals can also help reduce the severity and duration of symptoms. However, these medications are only effective if taken within the first 48 hours of symptoms appearing and only decrease the length of the illness by about one day. Tamiflu is most effective for people with an increased risk of flu complications. Those people include:

  • People age 65 or older
  • Young children
  • Pregnant people
  • People with chronic illnesses (diabetes, heart disease, asthma)
  • People with a BMI of 40 or higher

Potential complications can be easily treatable, like sinus or ear infections, or extremely dangerous, like developing pneumonia or other secondary bacterial infections. Complications can also make chronic medical conditions like congestive heart failure or diabetes worse. These complications can result in death for at-risk patients with compromised immune systems, such as those listed above. According to the CDC, over 21,000 lives are lost to the flu each year, with a high of 61,000 for the last decade during the 2017–2018 flu season.

Treating the flu

Once infected with influenza, proper self-care is key to a faster recovery. Getting plenty of rest and fluids and taking fever-reducing medicines like acetaminophen, ibuprofen, or naproxen can help.

Pandemics & vaccinations

While most flu viruses don’t cause pandemics, uncontrolled outbreaks can and do happen. The most recent flu pandemic was the 2009 Swine Flu (H1N1) outbreak. Flu pandemics are the result of novel Influenza A viruses. “Novel” means the virus has caused a human infection but is genetically different from the seasonal viruses currently circulating. Novel viruses originate in animals, such as pigs and birds, which is how terms like “swine flu” and “bird flu” or “avian flu” are derived.

Because of the genetic differences between novel and seasonal flu viruses, already-developed vaccines offer little protection. Vaccines do keep viruses that have caused pandemics in the past at bay. Each year the “flu shot,” as it is sometimes called, includes one influenza A(H1N1), one influenza A(H3N2), and one or two influenza B viruses. The strains chosen are scientists’ best estimates of what might be circulating in a given year. (Note: flu vaccines do not protect against influenzas type C & D.)

Influenza vs. COVID (and other respiratory illnesses)

In addition to the flu and COVID-19, other viruses can cause the same respiratory symptoms. These include rhinovirus, which is often called the “common cold,” and respiratory syncytial virus (RSV), which can be especially dangerous for children and older adults. Proper diagnosis of a respiratory virus is critical for receiving the right treatment plan and taking appropriate precautionary measures. For example, COVID-19 is more contagious than the flu. It spreads easily and is contagious for a longer period, and it also causes a much more severe illness in some people.

The other “flu”

It’s worth mentioning that sometimes when people say they have the flu, they mean they have a stomach bug. This bug is called viral gastroenteritis and is not the same as the flu. Gastroenteritis has many causes, the norovirus being one. It affects the digestive system and causes an intestinal infection marked by watery diarrhea, abdominal cramps, nausea or vomiting, and sometimes fever. Influenza, on the other hand, is a respiratory illness.

Key takeaways

Here are the key points to remember about the flu:

When is flu season? Flu season is referred to as the period of time when the virus is most active, between October and May, with peak activity from December to February.

How do I avoid the flu? Like the recommended precautions for COVID-19, frequent hand-washing and social distancing help prevent the flu spread. A yearly flu shot can prevent infection altogether or reduce the severity and longevity of symptoms.

How can I treat the flu? Rest, fluids, and fever-reducing medication can lessen flu symptoms. Antiviral medications like Tamiflu can help if taken within the first 48 hours from the onset of symptoms.

How long does the flu last? The flu can last between 7-10 days

How long is the flu contagious? The flu is contagious up to a week after the onset of symptoms, so infected people should avoid contact with the public.

How many people die from the flu each year? Over 20,000 people die from the flu every year.

How is the flu different than coronavirus? While both are respiratory illnesses, the flu and COVID-19 are caused by different viruses. (The flu by influenza, and COVID-19 by a coronavirus, SARS-CoV-2.) COVID-19 spreads more easily, can bring about a more severe illness, and results in a higher number of deaths than the flu.

For more information on either the flu or COVID-19, visit www.cdc.gov.