Flu (Influenza)
A contagious respiratory illness caused by influenza viruses that can range from mild disease to pneumonia, hospitalization, and death.
What causes Flu (Influenza)?
Flu is caused mainly by influenza A and B viruses. It spreads through respiratory particles released when an infected person breathes, talks, coughs, or sneezes, especially at close range and in crowded indoor spaces.
Symptoms and what to notice
Symptoms often begin suddenly and can include fever or chills, cough, sore throat, runny or stuffy nose, muscle aches, headache, marked fatigue, and sometimes vomiting or diarrhea, especially in children.
How long can it last?
Symptoms usually begin about 1 to 4 days after infection. Most uncomplicated illnesses improve in 3 to 7 days, although cough and fatigue can last two weeks or longer. People can spread flu before symptoms begin and for several days afterward.
How serious is Flu (Influenza)?
Flu can cause viral or bacterial pneumonia, worsening of asthma or heart disease, dehydration, encephalopathy, myocarditis, sepsis, hospitalization, and death. Risk is higher in young children, adults 65 and older, pregnant people, and people with certain chronic or immune conditions.
Treatment and management
Most mild cases need rest, fluids, and symptom care. Prescription antivirals work best when started early and are especially important for hospitalized, severely ill, or high-risk patients. Options include oseltamivir, zanamivir, peramivir, and baloxavir, selected according to age, pregnancy, health conditions, and clinical guidance.
Vaccine: who and when?
U.S. 2026-2027 flu vaccines are trivalent and are recommended annually for everyone age 6 months and older without a contraindication. Products include inactivated vaccines such as Fluarix, FluLaval, Fluzone, and Afluria; cell-based Flucelvax; recombinant Flublok; nasal-spray FluMist; adjuvanted Fluad; and Fluzone High-Dose. Adults 65 and older should preferentially receive high-dose, adjuvanted, or recombinant vaccine when available.
Cost note: Patient cost varies by product, insurance, pharmacy or clinic, and eligibility. Many U.S. insurance plans cover recommended annual flu vaccination without cost sharing when given in network.
Trusted source
Patient cost varies by insurance, provider, location, and eligibility. Coverage and public-health access differ, so listed prices are examples rather than guaranteed patient prices. This page is for general education and does not replace individualized medical advice, diagnosis, vaccination assessment, or treatment.
Flu (Influenza): Symptoms, Causes, Treatment, Prevention, and Vaccine Information
Seasonal influenza is not simply a severe cold. Influenza viruses infect the respiratory tract and cause annual epidemics that affect every age group. Most people recover at home, but the same infection can lead to pneumonia or destabilize chronic heart, lung, metabolic, or neurological disease. Influenza changes continually, which is why vaccine composition is reviewed every year and why vaccination is repeated each season. The 2026-2027 U.S. vaccines are trivalent and contain updated influenza A(H1N1), A(H3N2), and B/Victoria components.
Important: This page is for general education. It does not replace diagnosis, treatment, vaccination advice, or emergency care from a qualified healthcare professional.
Quick Facts
| Topic | Current information |
|---|---|
| Cause | Flu is caused mainly by influenza A and B viruses. It spreads through respiratory particles released when an infected person breathes, talks, coughs, or sneezes, especially at close range and in crowded indoor spaces. |
| Main symptoms | Symptoms often begin suddenly and can include fever or chills, cough, sore throat, runny or stuffy nose, muscle aches, headache, marked fatigue, and sometimes vomiting or diarrhea, especially in children. |
| Typical course | Symptoms usually begin about 1 to 4 days after infection. Most uncomplicated illnesses improve in 3 to 7 days, although cough and fatigue can last two weeks or longer. People can spread flu before symptoms begin and for several days afterward. |
| Treatment | Most mild cases need rest, fluids, and symptom care. Prescription antivirals work best when started early and are especially important for hospitalized, severely ill, or high-risk patients. Options include oseltamivir, zanamivir, peramivir, and baloxavir, selected according to age, pregnancy, health conditions, and clinical guidance. |
| Vaccine status | U.S. 2026-2027 flu vaccines are trivalent and are recommended annually for everyone age 6 months and older without a contraindication. Products include inactivated vaccines such as Fluarix, FluLaval, Fluzone, and Afluria; cell-based Flucelvax; recombinant Flublok; nasal-spray FluMist; adjuvanted Fluad; and Fluzone High-Dose. Adults 65 and older should preferentially receive high-dose, adjuvanted, or recombinant vaccine when available. |
History and Public-Health Importance
Influenza pandemics and seasonal epidemics have shaped modern public health. The 1918 pandemic caused extraordinary global mortality, while later pandemics occurred in 1957, 1968, and 2009. Surveillance now combines laboratory testing, outpatient illness reports, hospitalization data, and viral sequencing. This system helps identify emerging variants, guide antiviral policy, and select vaccine strains months before each season. Vaccination, early antiviral treatment, respiratory hygiene, ventilation, and protection of high-risk people remain the central tools for reducing severe outcomes.
What Causes Flu (Influenza)?
Flu is caused mainly by influenza A and B viruses. It spreads through respiratory particles released when an infected person breathes, talks, coughs, or sneezes, especially at close range and in crowded indoor spaces.
People transmit influenza mainly through respiratory particles at close range. Transmission can occur before a person realizes they are sick, and children or immunocompromised people may remain contagious longer. Touching contaminated surfaces is a less important route but can contribute when hands then touch the eyes, nose, or mouth. Influenza A viruses also circulate in animals, but ordinary seasonal flu is sustained primarily by person-to-person spread. Staying home while acutely ill, improving indoor air, washing hands, and covering coughs reduce exposure.
Symptoms and Typical Course
Symptoms often begin suddenly and can include fever or chills, cough, sore throat, runny or stuffy nose, muscle aches, headache, marked fatigue, and sometimes vomiting or diarrhea, especially in children.
Sudden fever, aches, headache, cough, and exhaustion are characteristic, but not everyone has fever. Vomiting and diarrhea occur more often in children. Uncomplicated illness usually improves within a week. Warning signs include breathing difficulty, chest pain, dehydration, confusion, seizures, bluish lips, severe weakness, or improvement followed by renewed fever and cough. Infants may show poor feeding, reduced alertness, or breathing problems rather than classic symptoms.
Complications and Who Is at Higher Risk
Flu can cause viral or bacterial pneumonia, worsening of asthma or heart disease, dehydration, encephalopathy, myocarditis, sepsis, hospitalization, and death. Risk is higher in young children, adults 65 and older, pregnant people, and people with certain chronic or immune conditions.
Primary viral pneumonia can progress quickly. Secondary bacterial pneumonia may appear after initial improvement and can be caused by organisms such as Streptococcus pneumoniae or Staphylococcus aureus. Flu can trigger asthma attacks, COPD exacerbations, heart attacks, strokes, encephalopathy, muscle inflammation, and sepsis. Pregnancy and the early postpartum period increase complication risk. Vaccination and prompt antiviral treatment are especially important for these groups.
How It Is Diagnosed
A clinician may diagnose flu from symptoms and local activity, but molecular tests can confirm influenza A or B. Rapid antigen tests are faster but less sensitive, so a negative result does not always exclude infection. Testing is most useful when it will guide treatment, infection-control decisions, or evaluation of a severe case. During periods when several respiratory viruses circulate, testing can distinguish influenza from COVID-19 and other infections.
Treatment and Recovery
Most mild cases need rest, fluids, and symptom care. Prescription antivirals work best when started early and are especially important for hospitalized, severely ill, or high-risk patients. Options include oseltamivir, zanamivir, peramivir, and baloxavir, selected according to age, pregnancy, health conditions, and clinical guidance.
Antivirals should begin as soon as possible for hospitalized patients, severe or progressive disease, and people at increased risk; treatment should not wait for a test result when clinical suspicion is high. Oseltamivir is widely used and is recommended for many hospitalized patients. Zanamivir is inhaled and is generally avoided in people with asthma or COPD because of bronchospasm risk. Peramivir is intravenous, while baloxavir is an oral single-dose option for selected uncomplicated cases. Antibiotics do not treat influenza itself but may be needed for a bacterial complication.
Vaccine Information
U.S. 2026-2027 flu vaccines are trivalent and are recommended annually for everyone age 6 months and older without a contraindication. Products include inactivated vaccines such as Fluarix, FluLaval, Fluzone, and Afluria; cell-based Flucelvax; recombinant Flublok; nasal-spray FluMist; adjuvanted Fluad; and Fluzone High-Dose. Adults 65 and older should preferentially receive high-dose, adjuvanted, or recombinant vaccine when available.
Children receiving influenza vaccine for the first time may need two doses separated by at least four weeks, depending on age and vaccination history. FluMist is a live attenuated nasal vaccine for eligible nonpregnant people within its licensed age range; injectable alternatives are used when it is unsuitable. For adults 65 and older, Fluad, Fluzone High-Dose, or Flublok is preferred when available, though any age-appropriate vaccine is better than delaying vaccination. Protection develops over roughly two weeks and decreases over time, so annual vaccination is necessary.
Prevention
Get the current seasonal vaccine, avoid close contact when ill, stay home until symptoms are improving and fever has been absent for at least 24 hours without fever-reducing medicine, clean hands, cover coughs, and improve ventilation. High-risk people should contact a clinician promptly after symptoms begin because antiviral benefit is greatest early. Masks can add protection in crowded indoor settings or when caring for someone who is sick.
When to Seek Medical Care
Seek urgent care for trouble breathing, persistent chest pain, confusion, seizures, severe dehydration, inability to wake, bluish discoloration, or rapidly worsening illness. Parents should seek immediate care for an infant with breathing difficulty, poor feeding, dehydration, or marked lethargy. High-risk people should call early even when symptoms initially seem mild.
Frequently Asked Questions
Is flu the same as a cold?
No. Both affect the respiratory tract, but influenza more often causes abrupt fever, aches, and marked fatigue and is more likely to cause serious complications.
Can the flu vaccine cause flu?
Injected flu vaccines cannot cause influenza. FluMist contains weakened viruses designed to replicate only in the cooler nasal passages and does not cause typical influenza illness.
Why is a flu vaccine needed every year?
Circulating viruses change, vaccine strains are updated, and immunity decreases over time. Annual vaccination restores protection for the current season.
Do antibiotics treat flu?
No. Antibiotics treat bacteria, not influenza viruses. They are used only when a clinician identifies or strongly suspects a bacterial complication.
Key Takeaway
Flu is common but not harmless. Annual vaccination, early antiviral treatment for high-risk or severe illness, and practical respiratory precautions can prevent many hospitalizations and deaths.
Medical, treatment, and vaccine information was reviewed against official U.S. public-health guidance available through September 2026.