What is flu

Last updated: April 1, 2026

Quick Answer: The flu (influenza) is a contagious respiratory illness caused by influenza viruses that spreads through respiratory droplets. It causes fever, cough, body aches, and fatigue, with most people recovering within 1-2 weeks.

Key Facts

Overview

Influenza, commonly called the flu, is a contagious respiratory illness caused by influenza viruses. Unlike the common cold, the flu typically develops suddenly and can be more severe. The illness spreads easily from person to person through respiratory droplets released when an infected person coughs, sneezes, or talks.

Transmission and Spread

The flu spreads primarily through respiratory droplets, which can travel up to 6 feet from an infected person. People can also contract the flu by touching surfaces contaminated with the virus and then touching their face. The incubation period—the time between exposure and symptom onset—is typically 1-4 days, though some people develop symptoms more slowly. People are most contagious from one day before symptoms appear through 5-7 days after onset.

Symptoms and Severity

Common flu symptoms include:

Most people experience symptoms for about one week, though cough and fatigue may persist longer. Severe cases can develop into pneumonia or require hospitalization.

Types of Influenza

Three main types of influenza viruses affect humans: Type A is the most common and causes seasonal epidemics; Type B is milder but can still cause significant illness; Type C is rare and typically causes mild illness. Influenza viruses constantly mutate, which is why immunity from previous infections or vaccines may not protect against new strains.

Prevention and Treatment

The flu vaccine is the most effective prevention method, available in multiple formulations. Annual vaccination is recommended, especially for high-risk groups like elderly individuals, young children, and people with chronic conditions. Treatment focuses on rest, hydration, and managing symptoms. Antiviral medications like oseltamivir can reduce symptom duration if taken early. Most people recover without specific treatment.

Related Questions

What is the difference between the flu and a cold?

The flu develops suddenly with high fever and severe body aches, while a cold develops gradually with milder symptoms. Flu symptoms are typically more severe and last longer, with greater risk of complications. Colds are caused by different viruses and are generally less serious than influenza.

How long does the flu last?

Most flu symptoms last about one week, though some people experience fatigue and cough for longer. Fever typically subsides after 3-4 days. Full recovery may take 1-2 weeks, and some symptoms like fatigue can persist for several weeks.

How do you prevent the flu?

The most effective prevention is the annual flu vaccine. Additionally, wash your hands frequently, avoid touching your face, stay home when sick, and maintain distance from sick people. Using hand sanitizer and cleaning frequently-touched surfaces also helps reduce transmission.

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Sources

  1. CDC - Influenza (Flu) Public Domain
  2. Wikipedia - Influenza CC-BY-SA-4.0