Key points
- Flu is a contagious respiratory illness caused by influenza viruses
- Flu can cause mild to severe illness. Some people are at higher risk for more serious flu illness.
- Most experts believe that influenza viruses spread mainly by droplets made when people with flu cough, sneeze, or talk
- Getting a flu vaccine has been shown to reduce the risk of flu and serious flu complications.
Understanding Influenza
Flu is a contagious respiratory illness caused by influenza viruses that infect tissues in the nose, throat, and sometimes the lungs. It can cause mild to severe illness, and at times can lead to death. Getting a flu vaccine has been shown to prevent flu and its severe complications. Flu vaccines are updated yearly.
Symptoms
Flu can cause mild to severe illness, and at times can lead to death. Flu symptoms usually come on suddenly. People who have flu often feel some or all of these signs and symptoms:
- fever* or feeling feverish/chills
- cough
- sore throat
- runny or stuffy nose
- muscle or body aches
- headaches
- fatigue (tiredness)
- some people may have vomiting and diarrhea, though this is more common in children than adults.
*It’s important to note that not everyone with flu will have a fever.
Not everyone with flu has symptoms
How long it takes for signs to show
Period of Contagiousness
You may be able to spread flu to someone else before you know you are sick, as well as when you have symptoms.
Studies have shown:
- People with flu are most contagious during the first three days of their illness.
- Some otherwise healthy adults may be able to infect others beginning one day before symptoms develop and five to seven days after becoming sick.
- Some people, including young children and people with weakened immune systems, may be contagious for longer periods of time.
Onset of Symptoms
The time from when a person is exposed and infected with influenza virus to when symptoms begin is about two days but can range from about one to four days.
People at risk
Anyone can get flu (including healthy people), and serious problems related to flu can happen to anyone, but some people are at higher risk of developing serious flu-related complications if they get sick. This includes people 65 years and older, people of any age with certain chronic medical conditions (such as asthma, diabetes, or heart disease), people with a body mass index (BMI) of 40 kg/m2 or higher, pregnant women, and children younger than 5 years, especially those younger than 2 years.
How it spreads
Most experts believe that flu viruses spread mainly by droplets made when people with flu cough, sneeze, or talk. These droplets can land in the mouths or noses of people who are nearby. Less often, a person might get flu by touching a surface or object that has Influenza virus on it and then touching their own mouth, nose or possibly their eyes.
Prevention
Flu vaccine has been shown to reduce the risk of flu-related illnesses and potentially serious flu complications that can result in hospitalization or even death. Flu vaccines are updated each year and annual vaccination is recommended. CDC also recommends other preventive actions (like staying away from people who are sick (distancing), covering coughs and sneezes, frequent handwashing, and taking steps for cleaner air) to help slow the spread of germs that cause respiratory (nose, throat, and lungs) illnesses like flu. More information is available about core and additional prevention strategies.
REFERENCE: https://www.cdc.gov/flu/about/index.html, accessed March 2026
WHO recommends that trivalent vaccines for use in the 2026 southern hemisphere influenza season contain the following:
Egg-based vaccines
- an A/Missouri/11/2025 (H1N1)pdm09-like virus;
- an A/Singapore/GP20238/2024 (H3N2)-like virus; and
- a B/Austria/1359417/2021 (B/Victoria lineage)-like virus.
Consistent with the four previous WHO recommendations since September 2023, it remains the opinion of the WHO influenza vaccine composition advisory committee that the inclusion of a B/Yamagata lineage antigen is no longer warranted.
Quadrivalent vaccines, where the transition to trivalent vaccines is not yet complete, contain a 4th component – a B/Yamagata lineage virus (B/Phuket/3073/2013-like virus).
There will no longer be updated recommendations for the B/Yamagata lineage component.
WHO has also updated recommendations for the development of new candidate vaccine viruses for zoonotic influenza, with a view on pandemic preparedness.
