Anju Goel, MD, MPH, is a board-certified physician who specializes in public health, communicable disease, diabetes, and health policy.
The flu is a highly contagious common illness caused by the influenza virus. Symptoms include fever, headache, body aches, runny nose, sore throat, cough, and fatigue that come on quickly. While most healthy people recover from the flu in about a week, children, the elderly, and people with weakened immune systems or chronic medical conditions are at greater risk of serious complications, including pneumonia and even death.
Two types of influenza viruses cause illness in humans: types A and B. Each type has many strains that mutate often, which is why people continue to come down with the flu year after year—and why flu shots only provide protection for one flu season. You can get the flu at any time of the year, but in the United States, flu season peaks between December and March.
Symptoms of the flu tend to come on suddenly over the course of a few hours and commonly last for four to five days. However, some people may recover from the flu in as few as two days or as many as seven or more days.
Influenza is contagious from before you even start to feel symptoms and continues for several days. On average, people are contagious 24 hours before symptoms appear for up to about five days after becoming ill.
Symptoms of influenza include fever, chills, headache, body aches, runny nose, sore throat, cough, and fatigue. Symptoms tend to come on quickly over the course of a few hours.
You can catch the influenza at any time of year, but it is most prominent in the fall and winter in the United States. Flu season typically spans from October through April with peak cases between December and March, although flu activity can last as late as May.
Fever is a common symptom with influenza. Part of the body’s defenses against the virus, an increased body temperature helps to prevent the virus from replicating. However, not everyone will experience a fever when they have the flu.
Influenza is passed from person to person through infected respiratory droplets shared via sneezing, coughing, talking, or blowing one’s nose. It can also be transmitted via contact with contaminated surfaces. The best way to prevent contracting the flu is to avoid close contact with people who are sick, wash your hands frequently, and get an annual flu shot.
The incubation period is the time between exposure to an infection and the first appearance of symptoms. The incubation period for influenza is between one and four days after exposure.
Influenza A is the most common type of influenza virus that causes the seasonal flu. Influenza A viruses are classified by subtypes based on surface proteins hemagglutinin (H) and neuraminidase (N). The viruses are named for the combination of proteins, such as A(H1N1) or A(H3N2).
Influenza B is the second most common type of influenza virus that causes the seasonal flu. Influenza B viruses are classified by subtypes based on lineage as B/Yamagata or B/Victoria.
Polymerase chain reaction (PCR) testing is a laboratory technique that produces millions of copies of a short segment of DNA until there is enough for analysis and detection. PCR testing is a diagnostic tool used to detect infections including influenza, coronavirus, HIV, hepatitis, and salmonella. It is also used to diagnose cancer, central nervous system diseases, and autoimmune diseases.
Vaccines are used to boost immunity against serious infectious diseases. Also called inoculation, a vaccine introduces a small piece of virus that is changed or killed so it can’t cause disease. The immune system produces antibodies against the pathogen, helping to fight against infection if you are exposed to it later. You may still catch the illness, but it will likely be a milder infection.
A viral infection is an infection caused by a virus. Viruses are microscopic organisms that can be transmitted from person to person. Some viruses, like the common cold, influenza, and COVID-19, can cause upper respiratory infection, while enteroviruses commonly cause vomiting and diarrhea.
Centers for Disease Control and Prevention. About flu. Updated September 5, 2019.
Centers for Disease Control and Prevention. Key facts about influenza (flu). Updated September 13, 2019.
Adam MP, Ardinger HH, Pagon RA, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2020. GeneReviews Glossary.
Centers for Disease Control and Prevention. How flu spreads. Updated August 27, 2018.
Centers for Disease Control and Prevention. The flu season. Updated July 12, 2018.
Centers for Disease Control and Prevention. Types of influenza viruses. Updated November 18, 2019.
Centers for Disease Control and Prevention. Flu symptoms & complications. Updated August 31, 2020.
Ghebrehewet S, MacPherson P, Ho A. Influenza. BMJ. 2016;355:i6258. Published 2016. doi:10.1136/bmj.i6258
U.S. National Library of Medicine. Vaccines (immunizations). MedlinePlus. Updated October 8, 2020.
U.S. National Library of Medicine. Viral infections. MedlinePlus. Updated October 19, 2020.