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“The flu” is a type of respiratory infection caused by influenza viruses. It has a lot in common with the common cold, but its symptoms tend to come on faster and be more severe. It isn’t typically serious, but it can be severe in newborns, older people, and people with immune issues.

Our evidence-based analysis on flu features 67 unique references to scientific papers.

Research analysis led by and reviewed by the Examine team.
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Summary of Flu

What is the flu?

“The flu” is a general term for respiratory infections caused by influenza viruses. Influenza viruses are constantly changing, which means individuals can contract the flu repeatedly. Although the flu isn’t often serious in the general population, newborns, older people, and people with immune issues can develop severe (and even life-threatening) illness.[1][2]

What are the main signs and symptoms of the flu?

Signs and symptoms of the flu typically include:

  • Fever and/or chills

  • Cough

  • Sore throat

  • Runny or stuffy nose

  • Muscle/body aches

  • Headaches

  • Fatigue

The flu shares many symptoms with the common cold. Compared to the common cold, symptoms tend to come on more rapidly and be more severe.[1]

How is the flu diagnosed?

Most of the time, the flu is self-diagnosed and individuals never seek medical treatment. In a clinical setting, diagnosis typically involves taking a patient history and possibly performing tests (some tests provide “rapid” results that are available in less than 1 hour; some, more accurate, tests require several days).[1] Additionally, clinicians will seek to rule out other, similar conditions, such as severe acute respiratory syndrome coronavirus 2 (COVID-19), respiratory syncytial virus (RSV), the common cold, Middle East respiratory syndrome coronavirus (MERS-CoV), and bacterial pneumonia.

What are some of the main medical treatments for the flu?

Getting a flu vaccine is the most effective way to prevent the flu, but it isn’t effective for treating a preexisting flu infection. Antiviral drugs (e.g., Oseltamivir, Zanamivir, Peramivir, Baloxavir) can be used to reduce the duration and severity of the flu, especially if they are administered early in the course of the illness.[2]

Other medications, such asnonsteroidal anti-inflammatory drugs (NSAIDs) and acetaminophen, can also be used to manage symptoms like fever, headache, and body/muscle pain.

Have any supplements been studied for the flu?

Vitamin C, vitamin D, and zinc show moderate evidence for reducing the risk of contracting the flu as well as reducing the duration and severity of flu symptoms.[3]

What's the connection between diet and the flu?

Much like the common cold, diet is connected to the flu through immunity. Diets that are sufficient in energy, micronutrients, and macronutrients are important for maintaining a robust immune system, which will help reduce the risk and severity of common flu infections.[4] Flavonoids (plant compounds that have antioxidant and immunomodulatory properties; found in especially high quantities in tea, chocolate, capers, and oregano)[5]may be a noteworthy nutrient for this purpose.[6]

Are there any other treatments for the flu?

Basic health hygiene habits, such as covering one’s cough and washing one’s hands, are important for preventing the spread of the flu.[1]

What causes the flu?

The flu is caused by influenza viruses, of which there are two types (A and B). Both influenza A and B viruses circulate throughout populations of humans, and are the typical cause of ordinary, seasonal spikes in flu rates. Influenza A viruses are also found in other animals — if they transfer to humans, they can cause flu pandemics, such as bird or swine flu.[2]

Frequently Asked Questions and Articles on Flu

Which supplements can help against colds and the flu?
Vitamin C, vitamin D, zinc, and other supplements may provide an edge against colds and the flu, but they should only serve to complement your main defensive arsenal: good hygiene, proper hydration, healthy diet, restful sleep, stress control, and exercise.
The downsides of caffeine intake
Caffeine can have a detrimental impact on your blood pressure, eye pressure, and acid reflux.
Can you boost your flu shot with prebiotics and probiotics?
Flu season is here, and flu vaccine efficacy may be low this year compared to last. But there could be a way to boost that efficacy: probiotics and/or prebiotics. A recent paper summarized the results of nine trials, and found promising results, although there is much yet to be learned.
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