Conditions guide

Influenza (Flu)

Influenza, or the flu, is a contagious respiratory illness caused by influenza viruses. It can range from mild to severe and, in some people, can lead to serious complications.

Last updated 3 sources

At a Glance

  • Flu is caused by influenza viruses that infect the nose, throat, and sometimes the lungs.
  • In the US, flu activity usually peaks between late fall and early spring.
  • Flu symptoms often start suddenly and may include fever, cough, and body aches.
  • Yearly flu vaccination is recommended by the CDC for most people 6 months and older.
  • Prescription antiviral medicines are available and tend to work best when started early.

What It Is

Influenza is a viral infection of the respiratory system. The main types that cause seasonal illness in people are influenza A and influenza B. Because these viruses change over time, people can get the flu more than once, and flu vaccines are updated each year.

Flu spreads mainly through tiny droplets made when people with flu cough, sneeze, or talk. Less often, a person may get flu by touching a surface with the virus on it and then touching their mouth, nose, or eyes. People can spread flu before they know they are sick.

Most people recover within a few days to under two weeks. However, flu can cause complications such as pneumonia, and can worsen long-term conditions like asthma or heart disease. Groups at higher risk include adults 65 and older, young children, pregnant people, and people with certain chronic health conditions.

Common Terminology

Influenza A and B
The two main types of influenza virus that cause seasonal flu outbreaks in people.
Antiviral medication
A prescription medicine that acts against a virus; flu antivirals can shorten illness and may reduce complications.
Flu vaccine
A vaccine updated each season to help protect against the flu viruses expected to circulate.
Incubation period
The time between infection and the start of symptoms, typically about one to four days for flu.
Pneumonia
An infection of the lungs that can be a complication of flu.
Rapid influenza diagnostic test
A quick test, usually on a nose or throat swab, that checks for flu virus proteins.

Common Causes and Associations

Flu is caused by infection with an influenza virus. Certain factors are associated with a higher chance of catching flu or developing complications.

  • Close contact with someone who has the flu, especially in homes, schools, and workplaces.
  • Crowded indoor settings during flu season.
  • Not being vaccinated against the current season's flu.
  • Age 65 or older, or being a young child, which is linked to a higher risk of complications.
  • Pregnancy, which is associated with more severe flu illness.
  • Chronic conditions such as asthma, heart disease, diabetes, kidney disease, or a weakened immune system.

Symptoms and Features

This is a general description of commonly reported symptoms, not a diagnostic checklist. Not everyone with flu has a fever, and flu can be hard to tell apart from other respiratory infections without testing.

  • Fever or feeling feverish, often with chills.
  • Cough and sore throat.
  • Runny or stuffy nose.
  • Muscle or body aches and headache.
  • Tiredness, which can be extreme and last for some time after other symptoms improve.
  • Vomiting and diarrhea, which are more common in children than adults.
  • Compared with a common cold, flu symptoms tend to start more suddenly and feel more severe.

Tests and Evaluation

Flu is often identified based on symptoms during flu season. Testing may be used when the result could change care or for public health tracking.

  • A swab from the nose or throat for a rapid flu test, which gives results quickly but can miss some infections.
  • Molecular tests, such as PCR, which are more accurate and may test for flu and other viruses like COVID-19 at the same time.
  • A physical exam, including listening to the lungs.
  • A chest X-ray in some cases, if pneumonia is a concern.
  • Blood tests or oxygen level checks in people who are more seriously ill.

Treatment Overview

Many people with flu recover with rest and supportive care. Treatment decisions, including whether antivirals are appropriate, are made with a healthcare professional.

  • Rest and drinking plenty of fluids are commonly recommended supportive measures.
  • Over-the-counter medicines are often used to ease fever and aches; questions about which are suitable can be discussed with a pharmacist or clinician.
  • Prescription antiviral medicines, such as oseltamivir, can shorten illness and are generally most helpful when started within about two days of symptoms.
  • Antivirals are especially considered for people who are very ill or at higher risk of complications.
  • Antibiotics do not work against flu viruses, but may be used if a bacterial complication develops.
  • Staying home while sick, covering coughs, and washing hands are commonly advised to reduce spread.

When Professional Care May Be Discussed

Public health agencies describe certain warning signs of severe flu illness. These are general examples, not a complete list.

  • Difficulty breathing, shortness of breath, or ongoing chest pain or pressure are generally treated as a medical emergency; in the US, people are advised to call 911.
  • New confusion, trouble waking up, seizures, or bluish lips or face are commonly cited emergency warning signs.
  • Symptoms that improve and then return with fever and worse cough are often described as a sign of a possible complication.
  • Signs of dehydration, such as not urinating, are a commonly cited concern, particularly in children.
  • People in higher-risk groups often contact a healthcare professional early in a flu illness to discuss whether antivirals are appropriate.
  • Worsening of a chronic medical condition during a flu illness is commonly discussed with a clinician.

Questions to Ask a Healthcare Professional

General questions people often find useful to bring to an appointment:

  • How can flu be told apart from a cold or COVID-19?
  • Who is considered at higher risk for flu complications?
  • How do flu antiviral medicines work, and when are they generally used?
  • Why does the flu vaccine need to be given every year?
  • How long is a person with flu generally considered contagious?
  • What are the commonly cited warning signs of severe flu illness?
  • Can a person get the flu even after being vaccinated?

Sources

This guide was written in plain language from the following authoritative references.

  1. Flu — MedlinePlus (U.S. National Library of Medicine)
  2. Influenza (Flu) — Centers for Disease Control and Prevention (CDC)
  3. Flu — NHS

Last updated October 8, 2026. FreeDoc guides are general educational information and are not a substitute for care from a qualified professional.