The flu (influenza) is a contagious respiratory infection caused by influenza viruses that attack the nose, throat, and lungs. It infects roughly one billion people worldwide each year, causing 3 to 5 million cases of severe illness and between 290,000 and 650,000 respiratory deaths annually. While most people recover within one to two weeks, the flu is significantly more dangerous than a common cold and can lead to serious complications in vulnerable groups.
How the Virus Infects Your Body
Influenza viruses come in four types: A, B, C, and D. Types A and B are the ones responsible for the seasonal flu epidemics that sweep through communities each winter. Type C causes only mild illness and doesn’t trigger epidemics. Type D primarily affects cattle and isn’t known to infect people at all.
When the virus reaches your airways, it latches onto cells lining your respiratory tract using a protein on its surface that binds to sugar molecules on your cells. Once attached, the cell absorbs the virus. Inside, the virus hijacks your cell’s machinery to copy its own genetic material, build new viral proteins, and assemble fresh virus particles that bud off the cell surface to infect neighboring cells. This cycle of invasion and replication is what triggers the immune response you experience as flu symptoms.
How the Flu Spreads
The virus travels between people through three main routes. When someone with the flu coughs or sneezes, they release a mix of particle sizes. Larger droplets (bigger than about 10 to 20 micrometers) fall to surfaces relatively quickly and land on nearby people. Smaller particles, under 5 micrometers, behave like true aerosols: they hang in the air and can be inhaled deep into the lungs. The third route is touching contaminated surfaces, since the virus can survive on hard, nonporous surfaces for up to 48 hours. Touching a doorknob or phone and then touching your face is enough to start an infection.
Symptoms and How They Progress
The flu hits fast. Unlike a cold, which creeps in gradually with a runny nose and mild sniffles, influenza symptoms tend to arrive all at once and with much more intensity.
A typical bout follows a predictable arc. During the first three days, you’ll likely experience a sudden fever, chills, headache, muscle pain, weakness, a dry cough, sore throat, and sometimes a stuffy nose. Around day four, the fever and muscle aches start to ease, but the cough, sore throat, and chest discomfort become more noticeable. Fatigue can feel heavy at this stage. By about day eight, most symptoms are fading, though the cough and tiredness often linger for one to two more weeks.
The easiest way to tell flu from a cold: colds are milder and more likely to center on nasal congestion and a runny nose. The flu brings fever, significant body aches, and a level of exhaustion that typically keeps you in bed. People with colds can often push through their day. People with the flu usually can’t.
Who Faces the Highest Risk
Most healthy adults recover from the flu without complications. But certain groups face a much higher chance of developing pneumonia, organ damage, or other dangerous outcomes. During recent flu seasons, 9 out of 10 people hospitalized with the flu had at least one underlying health condition.
The groups at greatest risk include:
- Adults 65 and older
- Children under 2, with the highest hospitalization and death rates among infants under 6 months
- Pregnant women, including up to two weeks after the end of pregnancy
- People with chronic conditions such as asthma, COPD, diabetes, heart disease, kidney or liver disorders, sickle cell disease, and weakened immune systems
- People with a BMI of 40 or higher
- People with neurological conditions or disabilities that affect muscle function, lung function, or the ability to cough and clear their airways
- Residents of nursing homes and long-term care facilities
Certain racial and ethnic groups, including non-Hispanic Black, Hispanic or Latino, and American Indian or Alaska Native populations, also face higher rates of flu hospitalization.
How the Flu Is Diagnosed
Doctors can often diagnose the flu based on symptoms alone during flu season, but testing confirms it. Rapid antigen tests return results in about 15 minutes by detecting viral proteins in a nasal swab. These tests are good at confirming a positive result (95 to 99 percent accuracy for true positives), but they miss a substantial number of infections. Their sensitivity sits around 50 to 70 percent, meaning they produce false negatives fairly often. The FDA now requires newer rapid tests to reach at least 80 percent sensitivity.
For a more accurate answer, some clinics use rapid molecular tests that deliver results within 30 minutes with much higher sensitivity. PCR-based tests remain the gold standard, though results take longer. All of these tests work best when performed within three to four days of symptom onset, while viral levels are still high.
Treatment and Timing
Antiviral medications can shorten the duration of illness and reduce the risk of serious complications, but timing matters. Treatment is most effective when started within 48 hours of the first symptoms appearing. After that window, antivirals still offer benefit for people with severe or worsening illness, including those who end up hospitalized.
The most commonly prescribed option is an oral antiviral (oseltamivir, often known by the brand name Tamiflu) taken twice daily for five days. A newer single-dose oral option (baloxavir, brand name Xofluza) works through a different mechanism and is convenient for people who prefer not to take a multi-day course. Inhaled and intravenous options also exist for specific situations. All four approved antivirals work against both influenza A and B.
For most people, treatment is straightforward rest, fluids, and over-the-counter fever and pain relief. Antivirals are typically reserved for those with confirmed flu who are at higher risk of complications or who are already severely ill.
Flu Vaccine Effectiveness
Annual vaccination remains the primary way to prevent the flu. Because influenza viruses mutate constantly, the vaccine is reformulated each year to match circulating strains, and its effectiveness varies from season to season.
Preliminary data from the 2024-2025 flu season in the United States shows the vaccine reduced outpatient flu visits by roughly 42 to 56 percent in adults, depending on the monitoring network. For children and adolescents, effectiveness ranged from about 59 to 63 percent across multiple studies, with protection against hospitalization reaching as high as 78 percent. Protection was generally stronger against the H1N1 strain (53 to 72 percent) than against H3N2 (16 to 55 percent), which is typical since H3N2 mutates more rapidly.
Even when the vaccine doesn’t prevent infection entirely, it consistently reduces the severity of illness. Vaccinated people who still catch the flu are less likely to end up in the hospital, less likely to need intensive care, and less likely to die from complications. For adults 65 and older, where outpatient effectiveness was more variable (18 to 51 percent depending on the study), the vaccine still cut hospitalization risk by 38 to 57 percent.