Key takeaways

The flu, caused by the influenza virus, can exhibit symptoms such as fever, chills, muscle aches, and fatigue, and it spreads mainly through contact with infected humans.
There are key differences between the flu and other illnesses like the common cold, COVID-19, and viral gastroenteritis (“stomach flu”), with distinctive symptoms for each.
While the flu generally resolves within a week, antiviral drugs, over-the-counter medications, hydration, and rest can help manage symptoms and reduce the duration of the illness.
The yearly flu vaccine is a crucial preventive measure that not only helps to avoid the flu but also reduces the severity of symptoms if you do get sick. The vaccine does not cause the flu.

There are 25 to 50 million cases of influenza per year in the U.S. alone, but not every wintertime illness is the flu. The flu is a specific type of respiratory infection caused by the influenza virus. The flu spreads via contact with an infected human, or rarely with an infected animal. It can be particularly severe for high-risk populations like older adults, young children, and people with chronic medical conditions. The flu can sometimes look similar to other illnesses like the common cold or coronavirus. But if you know which symptoms are associated with each infection, it’s much easier to recognize. And this is your one-stop flu symptom guide.

What are flu symptoms?

The flu virus comes with a variety of symptoms. Some of the earliest indicators include:

A runny or stuffy nose
A tickle in the throat
Mild body aches
Unusual tiredness
Sneezing

In most cases, these aren’t mild for very long. Symptoms of the flu can ramp up quickly and include:

Fever
Chills
Sore throat
Muscle aches
Headaches
Cough
Fatigue

Flu symptoms in children are often similar to those found in adults. However, children are more likely to experience a high-grade fever (103°F to 105°F) or gastrointestinal issues like nausea, vomiting, and diarrhea.

Illnesses manifest differently in different people, so it’s possible to have the flu with no respiratory symptoms, but it’s pretty rare.

Flu symptoms vs. “flu” misnomers

There are multiple types and strains of influenza, and it can be difficult to differentiate between them based on symptoms alone. The influenza virus has many categories and subtypes. There are four broad categories of influenza virus: influenza A, B, C, and D.

Influenza A and B are most common to cause the seasonal flu we typically worry about each year, and symptoms are generally so similar it would be nearly impossible to differentiate without laboratory tests for confirmation.

While Influenza C can also infect humans, the symptoms are generally very mild in comparison to infections with Influenza A and B.

Influenza D typically only infects cattle.

Be mindful that the term “flu” is a misnomer and often applied incorrectly. For example, the “stomach flu” is technically called viral gastroenteritis and is not a respiratory illness or related to influenza at all, but is caused by entirely different viruses like noroviruses or rotavirus.

Flu vs. flu misnomers
Common name	Flu	Stomach flu
Virus	Influenza A or B	Noroviruses, rotavirus
Common symptoms	Fever Chills Body aches Sore throat Headaches Fatigue	Abdominal pain Nausea Vomiting Diarrhea Chills Fatigue Loss of appetite Fever

Flu symptoms vs. the common cold

The common cold and the flu share a few symptoms, so they’re often confused. While both can cause a runny nose, sore throat, and cough, colds rarely cause fever, body aches, or exhaustion.

Flu vs. the common cold
Flu	Common cold
Fever Chills Body aches Sore throat Headaches Fatigue	Stuffy nose Sneezing Sore Throat Cough Congestion Low-grade fever

Flu symptoms vs. COVID-19

As the COVID-19 pandemic has swept through the United States—and the rest of the world—it has millions of people wondering whether they’ve caught coronavirus or the seasonal flu. The two have similar symptoms and methods of transmission, but coronavirus is more contagious, and its symptoms take longer to develop. The most significant difference is that COVID-19 more commonly causes shortness of breath and a loss of taste or smell.

Flu vs. COVID-19
Flu	Coronavirus
Fever Chills Body aches Sore throat Headaches Fatigue	Fever Cough Shortness of breath Muscle aches Fatigue Loss of taste or smell Runny nose

How long does it take for flu symptoms to appear? How long do they last?

Let’s look at a timeline. Symptoms can first appear one to four days after exposure to the virus (most often two). They usually last for five to seven days. Some symptoms, like fatigue, can linger for up to two weeks. Typically, someone with the flu is contagious for about a week starting one day before showing signs of illness. Getting the flu vaccine, however, can help shorten the flu’s duration and ultimately contagiousness.

This timeline can be longer for high-risk groups like older adults, young children, pregnant women, and people with chronic medical conditions or weakened immune systems. These groups are also at greater risk of complications, which can include:

Pneumonia
Ear infections
Sinus infections
Dehydration

Rarer, more severe flu complications include inflammation of the heart, brain, or muscles, and potential organ failure.

The common cold has a similar incubation period, just slightly faster. Symptoms can appear as early as 10 to 12 hours after infection, peak within one to three days, and last three to 10 days.

Coronavirus, on the other hand, develops slower. According to the Centers for Disease Control and Prevention, symptoms can appear two to 14 days after exposure and last as long as two weeks. To prevent its spread, the CDC recommends isolating for 10 days after the onset of symptoms and until fever-free for 24 hours without the use of fever-reducing medication, as long as all symptoms have improved.

How to treat flu symptoms

After contracting the flu virus, there’s no way to eliminate it completely. In most cases, it will simply run its course. However, there are treatments to help manage and mitigate the symptoms. For example:

Antiviral drugs: Since influenza is a viral infection, prescription antiviral medications like Tamiflu (oseltamivir phosphate) and Relenza (zanamivir) can reduce the duration and severity of its symptoms.
Over-the-counter pain relievers: Ibuprofen, acetaminophen, and naproxen can alleviate muscle aches and reduce fever.
OTC flu and cough medications: Cough and cold medicine like DayQuil can ease a cough, runny nose, sore throat, and other flu symptoms.
Fluids: Helps with hydration and breaks up mucus, making it easier to dispel. Good options are water, real fruit juices, tea, and sports drinks, but avoid alcohol or excessive caffeine.
Rest: Allows the body to focus on your immune system and helps avoid the exacerbation of fatigue and body aches. Staying home from work or social engagements also prevents the virus’ spread.
Humidifiers and steam: Can alleviate a stuffy nose and cough.
Cold compresses and tepid baths: This will help manage body temperature or at least keep the body feeling cool.

“I typically recommend rest and hydration with liquids containing electrolytes such as Gatorade or coconut water,” says Shirin Peters, MD, the founder of Bethany Medical Clinic in Manhattan. “For fever, I recommend Tylenol.” Dr. Peters also recommends self-isolation at the onset of flu-like symptoms since COVID-19 can exhibit nearly identical symptoms and may result in severe illness or (in some extreme cases) death.

When to go to the doctor for the flu

In most cases, the flu causes some discomfort for a few days but it ultimately resolves on its own within a week or so. But not always. For some, the flu can cause life-threatening, serious complications, as seen with older adults. The CDC estimates 70% to 85% of flu-related deaths are among people aged 65 and older. Anyone with one or more of the following risk factors for flu complications should visit a healthcare provider to prevent severe illnesses or infections:

Older than 65 years of age
Younger than 5 years of age, especially less than 2 years of age
Pregnancy
Obesity
Chronic medical conditions (asthma, cystic fibrosis, COPD, heart disease, kidney disease, etc.)
Compromised immune system (from HIV/AIDS or certain immunosuppressant treatments)

Otherwise, most people can weather the storm until symptoms end. Some symptoms do indicate a more dire situation and require medical care. According to Dr. Peters, anyone experiencing the following emergency symptoms should see a professional:

Shortness of breath
Abdomen or chest pain or pressure
Persistent dizziness, confusion, or drowsiness
Severe muscle pain
Severe weakness
Fever or cough that improve, then come back
Worsening of chronic medical conditions
Seizure
Lack of urination

People experiencing these symptoms, especially if they’re part of a high-risk population, should visit a healthcare provider as soon as possible.

Bottom line—flu symptoms are preventable

We’re not doomed to sit around waiting for flu season to arrive. The flu vaccine is an effective way to defend against it. Contrary to the common myth, a flu shot will not give someone the flu. Instead, it will catalyze the body’s creation of antibodies to fight influenza and reduce an infection’s severity.

A flu shot won’t prevent COVID-19 infections. But by protecting against the flu, it will hold off a virus that can weaken the immune system and make someone more susceptible to other infectious diseases, including coronavirus. In the end, it’s best to play it safe and get a flu shot.