Influenza, commonly known as the flu, is a contagious respiratory illness caused by influenza viruses. These viruses infect the nose, throat, and lungs, and can range in severity from mild to serious, sometimes even leading to fatal outcomes. Vaccination annually is the most effective way to prevent influenza and its complications.
Flu Symptoms: Recognizing the Signs
Influenza can manifest as a mild or severe illness, and in some cases, it can be life-threatening. Flu symptoms typically appear suddenly and may include a combination of the following:
- Fever* or feeling feverish with chills
- Cough
- Sore throat
- Runny or congested nose
- Muscle or body aches
- Headaches
- Fatigue (feeling very tired)
- Vomiting and diarrhea can occur, particularly in children, but are less common in adults.
*It is important to remember that not everyone with the flu will develop a fever.
Asymptomatic Flu Cases
Interestingly, not everyone infected with the influenza virus will show symptoms. Studies have indicated that a percentage of individuals who test positive for the flu virus do not experience any symptoms at all. A household study spanning several flu seasons from 2017-2023 revealed that approximately 8% of people who tested positive for influenza were asymptomatic.
Further Reading: To understand the nuances, explore the Similarities and Differences between Flu and COVID-19.
Contagious Period and Symptom Onset
How Long is the Flu Contagious?
Flu is contagious, and you can spread it to others even before you realize you are sick. You are also contagious while you are experiencing symptoms.
- Individuals with the flu are most contagious in the first 3 days after their illness begins.
- Generally healthy adults can be contagious starting from one day before symptoms appear and for up to 5 to 7 days after becoming sick.
- Certain populations, such as young children and those with weakened immune systems, may remain contagious for longer durations.
Flu Incubation Period
The incubation period, which is the time between exposure to the influenza virus and the start of symptoms, is approximately two days, but it can range from one to four days.
Who is at Risk from the Flu?
Anyone can contract influenza, regardless of their health status. While serious flu complications can occur at any age, certain groups are at a higher risk of developing serious flu-related complications if they get sick. These high-risk groups include:
- Adults aged 65 years and older
- Individuals of any age with specific chronic medical conditions like asthma, diabetes, or heart disease
- People with a body mass index (BMI) of 40 kg/m2 or higher (BMI Calculator)
- Pregnant women
- Children younger than 5 years old
How Does Influenza Spread?
The primary mode of influenza virus transmission is through respiratory droplets. These droplets are generated when an infected person coughs, sneezes, or talks. These virus-containing droplets can then land in the mouths or noses of people nearby, leading to infection. Less commonly, transmission can occur when a person touches a contaminated surface or object with the flu virus on it and subsequently touches their mouth, nose, or possibly their eyes.
Flu Prevention Strategies
The most effective way to prevent influenza is annual flu vaccination. Flu vaccines are designed to reduce the likelihood of flu illness and the risk of serious complications that can lead to hospitalization or even death. The Centers for Disease Control and Prevention (CDC) also recommends everyday preventive actions to minimize the spread of respiratory illnesses like the flu. These actions include:
- Avoiding close contact with sick individuals (physical distancing) (Physical Distancing Guidelines).
- Covering coughs and sneezes (Hygiene Practices).
- Frequent handwashing.
- Improving indoor air quality (Cleaner Air Measures).
More comprehensive information on prevention strategies is available regarding core and additional prevention strategies.
Key Facts About Influenza
Flu Infection Rates in the U.S. Annually
A 2018 CDC study published in Clinical Infectious Diseases investigated the percentage of the U.S. population affected by the flu each year. Using two different methodologies, the study found consistent results, indicating that, on average, approximately 8% of the U.S. population contracts the flu every season, with seasonal variations ranging from 3% to 11%.
Flu Susceptibility Across Age Groups
The same CDC study also revealed that children are the most susceptible age group to influenza infection, while adults aged 65 and older are the least likely to get sick with the flu. The median incidence rates by age group were:
- 9.3% for children aged 0-17 years
- 8.8% for adults aged 18-64 years
- 3.9% for adults aged 65 years and older
This data highlights that children under 18 are more than twice as likely to develop symptomatic influenza compared to older adults.
Estimating Seasonal Flu Incidence
Because not everyone with the flu seeks medical care or gets tested, the actual number of influenza infections each season is an estimation. These estimations are derived statistically from CDC-measured flu hospitalization rates, adjusted to project the total number of influenza infections in the U.S. for a given flu season. These infection estimates are then divided by the census population to determine the seasonal incidence or attack rate, representing how common flu infections are within the population.
Flu Season Severity and Incidence
The proportion of the population that contracts the flu varies from season to season. Research published in Clinical Infectious Diseases showed that between 3% and 11% of the U.S. population develops symptomatic flu each year. The 3% estimate corresponds to the 2011-2012 season, a low-severity H1N1-predominant season. In contrast, the estimated incidence reached around 11% during the 2012-2013 (moderate severity, H3N2-predominant) and 2014-2015 (high severity, H3N2-predominant) seasons.
Table 1. Estimates of the Incidence of Symptomatic Influenza by Season and Age-Group, United States, 2010–2022
| Season | Predominant Virus(es) | Season Severity | Incidence, %, by Age Group |
|---|---|---|---|
| 0-4 yrs | |||
| 2010-11 | A/H3N2, A/H1N1pdm09 | Moderate | 13.7 |
| 2011-12 | A/H3N2 | Low | 4.7 |
| 2012-13 | A/H3N2 | Moderate | 17.8 |
| 2013-14 | A/H1N1pdm09 | Moderate | 12.7 |
| 2014-15 | A/H3N2 | High | 16.1 |
| 2015-16 | A/H1N1pdm09 | Moderate | 11.0 |
| 2016-17 | A/H3N2 | Moderate | 11.9 |
| 2017-18 | A/H3N2 | High | 17.1 |
| 2018-19 | A/H1N1pdm09, A/H3N2 | Moderate | 15.2 |
| 2019-20 | A/H1N1pdm09, B | Moderate/High | 19.8 |
| 2020-21* | |||
| 2021-22 | A/H3N2 | Low | 4.6 |
| Median | 13.7 |
* The burden estimate for the 2020-2021 season was not calculated due to the uncharacteristically low level of flu activity that season.
Potential Flu Complications
Flu complications can be serious and include:
- Bacterial pneumonia
- Ear infections
- Sinus infections
- Worsening of pre-existing chronic medical conditions such as congestive heart failure, asthma, or diabetes.
Diagnosing Influenza
Differentiating influenza from other respiratory illnesses caused by viruses or bacteria based solely on symptoms is challenging. Diagnostic tests are available to confirm influenza infection.
Learn More: For detailed information, refer to Diagnosis for Flu.
Flu Treatment Options
Antiviral medications are available to treat influenza. These drugs can help shorten the duration of illness and reduce symptom severity.
Further Information: For comprehensive information about managing the flu, consult Information for the Flu Season.
Influenza Virus Types and Resources
Influenza A viruses are further classified into subtypes based on genes encoding surface proteins. Throughout a flu season, different types (A and B) and subtypes (for type A only) of influenza viruses circulate and cause illness.
For further reliable information and resources on influenza, please refer to reputable health organizations such as the Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO).
