What Are the First Signs of Flu?

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When you get the flu (influenza), it will often start with mild muscle aches, a slight tickle in the throat, or a worn-out feeling you just can't shake. Common wisdom suggests that if you treat flu when symptoms first appear, you may be able to shorten the duration and severity of the illness. In recent years, a number of prescription drugs have been approved for just such a purpose.

Even if you are unable to get these medications, by recognizing the early signs and symptoms of flu, you can act quicker—not only getting the bed rest you need, but to take steps to prevent the spread of infection to others.

first signs of the flu

Verywell / Ellen Lindner


Spotting the early signs and symptoms of flu can help you seek treatment that can shorten the course of an infection. The most common early symptoms are:

  • Sudden high fever (over 100.4 degrees F)
  • Chills
  • Generalized muscle aches
  • Malaise (a general feeling of unwellness)
  • Tiredness
  • Headache (usually extending across the forehead and behind the eyes)

Once full-fledged acute symptoms develop, there is really nothing you can do other than rest and let the disease run its course.


The flu is caused by a family of viruses that affect the respiratory system. Unlike colds, which can be caused by any number of different viruses (including rhinoviruses, adenoviruses, and coronaviruses), flu is caused exclusively by influenza viruses (designated as influenza type A, B, C, or D).

When flu symptoms first develop, it may be hard to differentiate them from the early signs of a cold. But, there are a few key ways to tell them apart.

Flu viruses almost invariably have a shorter incubation period, meaning that symptoms tend to develop fast and furiously, usually within a day or so. Because of this, the immune system will respond more aggressively, flooding the body with inflammatory compounds to help neutralize the virus.

This aggressive immune assault will manifest with symptoms that are usually more severe and/or less characteristic than the common cold.

Differences Between Colds and Flu
  Cold Symptoms Flu Symptoms
Incubation period 2 to 3 days 1 to 2 days
Onset of symptoms Gradually, between 4 to 7 days of exposure Rapidly, between 1 to 3 days of exposure
Fever Uncommon Common, usually lasts 3 to 4 days
Chills Uncommon Common
Headache Sometimes, usually related to sinus congestion Common, related more to whole-body inflammation
Sneezing and congestion  Common Occasionally
Cough Common, mild to moderate Common, often severe
Sore throat  Occasionally Common
Fatigue Occasionally, mild to moderate Common, often severe

Stages of Infection

Influenza develops in stages, the early stage of which is referred to as the incubation phase and the latter of which is known as the acute phase.

The incubation phase is the time following exposure before symptoms first appear. It is during this phase that the virus will actively replicate and, by doing so, trigger the release of defensive proteins called cytokines. Cytokines are pro-inflammatory, meaning that they spur the body to generate inflammation so that tissues and blood vessels can swell to accommodate larger immune cells (known as monocytes).

In the latter part of the incubation phase, subtle symptoms will start to develop. These early symptoms are referred to as prodromal and serve as a warning sign of a developing infection.

After the prodromal phase is the acute phase, which lasts for around two to three days with all of the "classic" symptoms of flu. Most flu symptoms will disappear after about a week, but a cough may persist for up to two weeks.

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The flu can usually be diagnosed by symptoms alone, particularly during flu season. This is especially true with the prodromal flu symptoms, during which treatment will often be dispensed presumptively. Even if a rapid influenza diagnostic test (RIDT) is available, the test is not all that sensitive and can regularly return false-negative results.

To prevent complications and the further spread of infection, certain prescription flu medications can be dispensed over the phone by a doctor or a telehealth physician without a face-to-face meeting.


Unlike the common cold, for which there are no drug treatments, the flu can be shortened by a day or two if certain antiviral drugs are taken within 24 to 48 hours of the first appearance of symptoms. The drugs work by slowing the replication of the virus and, by doing so, potentially reduce the length and severity of the illness.

Even so, the drugs don't always work, often because people miss the early signs and get treated too late. If not taken within the first 24 to 48 hours, the drugs will likely be of any benefit.

The four antiviral treatment options approved by the U.S. Food and Drug Administration (FDA) are:

  • Rapivab (peramivir), approved for adults and children 2 years and up
  • Relenza (zanamivir), approved for adults and children over 7 years
  • Tamiflu (oseltamivir), approved for adults and children over 12 months
  • Xofluza (baloxavir marboxil), approved for adults and children over 5 years

For people that are at high risk of flu complications (including young children, the elderly, pregnant women, and people with chronic health conditions), taking an antiviral drug can mean the difference between being hospitalized and recovering at home.

Relenza, Tamiflu, and Xofluza are not substitutes for the annual flu vaccine.

A Word From Verywell

The best way to deal with annual flu outbreaks is to avoid infection. The most effective strategy for doing so is getting an annual flu shot or FluMist (the nasal spray flu vaccine). The Centers for Disease Control and Prevention (CDC) recommends annual influenza vaccination for everyone 6 months and older, ideally by the end of October.

Frequently Asked Questions

  • Why is it important to get influenza diagnosed early?

    The severity and duration of flu symptoms can be reduced by taking antiviral drugs within the first 24 to 48 hours after symptoms begin. This is because the antiviral drugs work to slow the replication of the flu virus. This is especially important for people who are at high risk of flu complications.

  • What drugs are used to treat influenza?

    There are currently four antiviral treatments used to treat influenza: 

    • Rapivab (peramivir)
    • Relenza (zanamivir)
    • Tamiflu (oseltamivir)
    • Xofluza (baloxavir marboxil)

    Antiviral medications work best if taken in the first 48 hours after symptoms begin. 

    Over-the-counter cold and flu medications can help to relieve symptoms, but they don't shorten the course of the illness. Combination flu medications sold over the counter typically contain an analgesic, cough suppressant or expectorant, an antihistamine, and decongestants.

  • How quickly does the flu come on?

    The influenza virus has an incubation period of one to two days. Symptoms tend to come on rapidly over the course of a few hours. For instance, you may feel fine when you wake up in the morning, gradually feel worse throughout the day, and have full-blown flu symptoms by evening. Similarly, you might go to bed feeling fine and wake up feeling undeniably sick with body aches, headache, and fever. 

9 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  2. Centers for Disease Control and Prevention. Flu symptoms and complications.

  3. Lessler J, Reich NG, Brookmeyer R, et al. Incubation periods of acute respiratory viral infections: A systematic review. Lancet Infect Dis. 2009 May;9(5):291-300. doi:10.1016/S1473-3099(09)70069-6

  4. Centers for Disease Control and Prevention. Rapid influenza diagnostic tests.

  5. Shie, J., Fang, J. Development of effective anti-influenza drugs: congeners and conjugates – A reviewJ Biomed Sci. 2019;26:84. doi:10.1186/s12929-019-0567-0

  6. GlaxoSmithKline. Relenza (zanamivir) inhalation powder, for oral inhalation.

  7. Hoffmann-La Roche. Tamiflu complete product information sheet.

  8. Genentech, Inc. Genentech: Xofluza TM (baloxavir Marboxil) - Information for healthcare providers.

  9. Centers for Disease Control and Prevention. Who needs a flu vaccine and when?.

By Kristina Duda, RN
Kristina Duda, BSN, RN, CPN, has been working in healthcare since 2002. She specializes in pediatrics and disease and infection prevention.