Influenza

Influenza is the seasonal visitor that makes a common cold look polite.

By Greg Goodwin · Published November 4, 2026

Cheat Sheet

2024-25 burden (CDC, preliminary): 51 million illnesses, 23 million medical visits, 710,000 hospitalizations, and 45,000 deaths in the US.
Severity: 2024-25 was rated high severity for all ages, the first high severity season since 2017-18.
Age split: adults 65 and older accounted for 57% of flu hospitalizations and 71% of flu deaths that season.
Contagious window: some healthy adults can spread flu from 1 day before symptoms to 5 to 7 days after becoming sick; most contagious in the first 3 days.
Incubation: about 2 days from exposure to symptoms, with a range of about 1 to 4 days.
Vaccination impact: CDC estimates flu vaccination prevented about 180,000 hospitalizations and 12,000 deaths in 2024-25.

The 60-Second Version

Influenza, or flu, is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and sometimes the lungs. It ranges from mild to severe and can occasionally lead to death. Symptoms usually come on suddenly and can include fever or feeling feverish, chills, cough, sore throat, a runny or stuffy nose, body aches, headaches, and fatigue, although not everyone gets a fever. Most experts believe it spreads mainly through droplets made when infected people cough, sneeze, or talk, and a person can pass it on before feeling sick. Anyone can get it, but people 65 and older, young children, pregnant women, and people with certain chronic conditions face higher risk of complications. Vaccines are updated every year to match circulating viruses. This page is general information, not medical advice, so talk with a clinician about your own situation.

The Long Version

What the Virus Does

Flu is caused by influenza viruses that infect tissues in the nose, throat, and sometimes the lungs. CDC identifies two types, A and B, behind seasonal epidemics, and influenza A viruses are further divided into subtypes such as A/H1N1 and A/H3N2 based on their surface proteins. Influenza A viruses are also found in animals, including ducks, chickens, pigs, horses, whales, and seals. Viruses can change in two ways, called antigenic drift and antigenic shift, which is part of why a vaccine formula from one year may need updating the next. Flu symptoms usually arrive abruptly, which is one clue that separates it from many colds.

Spread and Timing

CDC says most experts believe flu viruses spread mainly by droplets that land in the mouths or noses of people nearby, and less often through touching a contaminated surface and then the face. The time from infection to symptoms is about two days, with a range of roughly one to four. People are most contagious during the first three days of illness, and some otherwise healthy adults can infect others starting one day before symptoms and for five to seven days after becoming sick. Young children and people with weakened immune systems may be contagious longer. Some infections cause no symptoms: a household study covering 2017 to 2023 found that 8 percent of people who tested positive had none.

Who Gets Very Sick

Anyone can get flu, but some people face higher risk of serious complications. CDC lists people 65 and older, people of any age with certain chronic conditions such as asthma, diabetes, or heart disease, people with a body mass index of 40 or higher, pregnant women, and children younger than 5, especially those younger than 2. Complications can include bacterial pneumonia, ear and sinus infections, and worsening of conditions like heart failure or asthma. Interestingly, getting sick and getting very sick are different questions: a CDC study found that children were most likely to get symptomatic flu and adults 65 and older were least likely, yet older adults dominate the hospitalization numbers.

A Look at 2024-25

CDC estimates that the 2024-25 season brought 51 million illnesses, 23 million medical visits, 710,000 hospitalizations, and 45,000 deaths in the United States, based on data from October 2024 through April 2025. The season was classed as high severity for all ages, the first since 2017-18. Adults 65 and older made up 57% of hospitalizations and 71% of deaths. CDC estimates 790 flu-related pediatric deaths, while 290 were actually reported as of February 14, 2026, the highest count for a regular season since reporting began in 2004. These burden figures are preliminary and will not be considered final until about 2027.

What Vaccination Adds

CDC says flu vaccination reduces the risk of flu illness and of serious complications, and it recommends annual vaccination alongside habits such as handwashing, covering coughs, and cleaner air. For 2024-25 CDC estimates vaccination prevented about 10 million illnesses, 5 million medical visits, 180,000 hospitalizations, and 12,000 deaths. Coverage varied by age, from 31% among adults 18 to 49 to 63% among those 65 and older. Effectiveness against outpatient visits ranged from 40% in adults 65 and older to 60% in children 6 months to 4 years. Antiviral drugs for treating flu exist, and a clinician can advise whether they fit a given case.

Why People Care

Flu returns every year, fills clinics and hospitals, and hits older adults, young children, and people with chronic conditions hardest. Knowing how it spreads and how long people stay contagious makes it easier to decide when to stay home. This is general information, not medical advice.

Glossary

Influenza A and B: The two types of flu virus responsible for seasonal flu epidemics.
Subtype: A category of influenza A virus based on its surface proteins, such as A/H1N1 and A/H3N2.
Antigenic drift and shift: The two ways influenza viruses can change, which is one reason flu vaccines are updated yearly.
Flu burden: CDC's modeled estimate of flu illnesses, medical visits, hospitalizations, and deaths in a season.
Droplets: Particles made when people with flu cough, sneeze, or talk, which are thought to be the main way flu spreads.

Go Deeper

More to Explore

🎲 Take Me Somewhere Random