Influenza, commonly called the flu, is a contagious respiratory infection caused by influenza viruses that infect the nose, throat, and lungs. It strikes suddenly, typically bringing fever, body aches, and exhaustion that feel noticeably worse than a regular cold. Most healthy people recover within one to two weeks, but the flu causes serious complications and hospitalizations every year, particularly in young children, older adults, and people with chronic health conditions.
Types of Influenza Virus
Four types of influenza virus exist: A, B, C, and D. Only A and B drive the seasonal flu epidemics that sweep through communities each winter. Influenza A is the most dangerous of the group. It’s the only type capable of causing pandemics, those worldwide outbreaks that emerge when a dramatically new strain appears and few people have immunity to it.
Influenza B circulates alongside A during flu season and can cause equally miserable illness, but it doesn’t trigger pandemics. Influenza C causes mild infections and doesn’t spark epidemics. Influenza D primarily affects cattle and isn’t known to make people sick.
How the Virus Spreads and Infects Cells
Flu spreads mainly through tiny droplets launched into the air when an infected person coughs, sneezes, or talks. You can also catch it by touching a contaminated surface and then touching your mouth, nose, or eyes. After exposure, symptoms typically appear about two days later, though the window ranges from one to four days.
People with the flu can spread the virus starting one day before they feel any symptoms, which is part of why it moves through households and workplaces so efficiently. You’re most contagious during the first three days of illness, though you can remain infectious for five to seven days. Young children and people with weakened immune systems may spread the virus for even longer.
At the cellular level, the flu virus uses two key proteins on its surface to do its work. The first acts like a key, latching onto sugars on the surface of cells lining your airways and triggering the cell to pull the virus inside. The second protein handles the exit. Once the virus has hijacked a cell and forced it to produce thousands of new virus copies, that second protein snips the chemical bonds holding the new viruses to the dying cell, freeing them to spread to fresh targets. It also helps the virus cut through the protective mucus layer in your airways, giving it easier access to cells underneath.
Symptoms and How Flu Differs From a Cold
Flu symptoms hit fast. One moment you feel fine, and within hours you may experience fever or chills, a cough, sore throat, runny or stuffy nose, muscle or body aches, headaches, and deep fatigue. Not everyone develops a fever, but the combination of sudden onset and full-body aches is a hallmark that separates flu from most other respiratory infections.
Colds and flu overlap enough in symptoms that telling them apart on feel alone can be tricky. A few patterns help. Colds come on gradually and tend to center on the nose and throat: sneezing, a runny nose, mild congestion. Flu arrives abruptly and hits the whole body. People with colds are more likely to have a stuffy or runny nose as their main complaint, while people with flu are more likely to feel wiped out with aches and fever. The biggest practical difference is that colds rarely lead to serious complications, while flu can progress to pneumonia, hospitalization, or worse.
Who Faces the Highest Risk
Certain groups are far more likely to develop dangerous complications from the flu. Age sits at the top of the list: adults 65 and older and children younger than 2 face the greatest risk, with infants under 6 months having the highest hospitalization and death rates among children. Pregnant women, including up to two weeks after delivery, also fall into the high-risk category.
A wide range of chronic conditions raises the stakes as well:
- Lung conditions like asthma, COPD, and cystic fibrosis
- Heart disease, including congestive heart failure and coronary artery disease
- Diabetes and other endocrine disorders
- Weakened immune systems from HIV, cancer, chemotherapy, or long-term steroid use
- Kidney or liver disorders
- Neurological conditions that affect muscle function, breathing, or the ability to clear the airways
- Obesity with a BMI of 40 or higher
- Blood disorders such as sickle cell disease
People living in nursing homes and long-term care facilities face elevated risk, as do certain racial and ethnic groups, including non-Hispanic Black, Hispanic or Latino, and American Indian or Alaska Native people, who experience higher rates of flu-related hospitalization.
Complications
The most common serious complication is pneumonia. The flu virus can cause pneumonia on its own, but it also sets the stage for secondary bacterial pneumonia by damaging the cells lining your airways and impairing your lungs’ ability to clear bacteria. The immune cells that normally patrol the lungs become less effective during a flu infection, and the tiny hair-like structures that sweep mucus and debris out of the airways stop working properly. The bacteria most likely to take advantage of this weakened state are the same ones that commonly live in the throat and nose.
Beyond the lungs, flu can trigger cardiovascular problems including heart attacks, heart failure, and inflammation of the heart muscle or its surrounding lining. Less common complications include muscle inflammation, seizures, brain swelling, stroke, and Guillain-Barré syndrome, a condition where the immune system attacks the nerves. Pulmonary complications are especially frequent in older adults and people with suppressed immune systems.
Diagnosis
Because flu symptoms overlap with colds and other respiratory viruses, a definitive diagnosis requires testing. The most common option in a doctor’s office is a rapid test that can return results in about 15 minutes. These tests must meet a minimum of 80% sensitivity and 95% specificity compared to the gold-standard molecular test (RT-PCR). In practical terms, a positive rapid test is highly reliable, but a negative result doesn’t completely rule out the flu, especially early in the illness. If your doctor suspects flu despite a negative rapid test, they may order the more accurate molecular test.
Treatment and Antiviral Timing
For most healthy people, flu treatment means rest, fluids, and managing symptoms with over-the-counter fever reducers and pain relievers. Antiviral medications are available for people at high risk of complications or anyone with severe illness. These drugs work best when started within 48 hours of the first symptoms. After that window, they can still help in serious cases, particularly for hospitalized patients, but the benefit is smaller.
The most widely prescribed antiviral is a twice-daily oral medication taken for five days. A newer option requires only a single dose, which can be more convenient. Both work by interfering with the virus’s ability to reproduce or escape from infected cells. Your doctor will decide whether antivirals make sense based on your risk profile and how far along your illness is.
Flu Season Timing
In the Northern Hemisphere, flu season runs roughly from October through March, with activity usually peaking between December and February. In the Southern Hemisphere, the pattern flips: flu circulates primarily from April through September, sometimes lingering into October or November. This predictable seasonal rhythm is why annual vaccination campaigns kick off each fall in countries like the United States.
Vaccine Effectiveness
The flu vaccine is reformulated every year to match the strains expected to circulate, but its protection varies significantly from season to season. For the 2025-26 season, vaccine effectiveness against flu-related outpatient visits was 38% to 41% among children and adolescents, and 22% to 34% among adults. Against hospitalization, it was 41% for children and adolescents and around 30% for adults.
Protection varies by strain too. The vaccine performed better against influenza B, reducing outpatient visits by 45% to 71% in children and 63% in adults. Against the influenza A(H3N2) strain, effectiveness was lower: 35% against outpatient visits and 38% against hospitalization in children. For adults 65 and older, effectiveness against outpatient visits ranged from 30% to 41% depending on the study network.
These numbers may look modest, but even partial protection reduces the severity of illness if you do get infected. A vaccinated person who catches the flu is less likely to end up in the hospital than an unvaccinated person with the same strain. For people in high-risk groups, that reduced severity can be the difference between recovering at home and a dangerous hospitalization.