Flu virus, explained

TL;DR:

Four flu virus types exist. A and B drive seasonal epidemics.
Flu spreads by droplets, hands, and close contact. Incubation is 1 to 4 days.
Annual vaccination reduces severe illness, especially for high-risk groups.
Antivirals like oseltamivir and baloxavir help when started early.
WHO updates vaccine composition twice a year for each hemisphere.

Influenza, or flu, is a contagious infection of the nose, throat, and lungs. It leads to mild to severe illness and can cause complications in some people. Annual activity occurs worldwide.

The viruses behind the flu

There are four types of influenza viruses, named A, B, C, and D. Types A and B cause seasonal epidemics in people each year. Types C and D rarely cause human illness.

Why types matter

Type A changes often and can spark larger waves. Type B also circulates and can hit children and teens hard. Knowing the circulating types guides vaccine design and treatment choices.

How flu spreads

Flu spreads when a sick person coughs or sneezes and releases droplets. Hands that touch the face or shared surfaces can pass the virus. Crowded indoor places speed spread. The incubation period is about 2 days, with a range of 1 to 4 days.

Common symptoms

Fever, cough, sore throat, runny or stuffy nose, body aches, headache, and fatigue are common symptoms. Vomiting and diarrhea can occur in children. Seek urgent care for trouble breathing, chest pain, confusion, or dehydration.

Who faces the highest risk

Some people are more likely to develop severe disease or complications.

Adults 65 years and older
Children under 5 years, especially under 2
Pregnant people and those up to 2 weeks after pregnancy
People with chronic conditions, such as heart or lung disease, diabetes, kidney disease, obesity, or weakened immunity
Residents of nursing homes, and healthcare workers who have frequent exposure

Annual vaccination is especially important for these groups.

Vaccines: your best defense

Why you need a shot every year

Flu viruses change over time. Immunity from last year’s infection or shot fades. A yearly shot boosts protection and reduces the risk of severe disease and hospitalization.

How vaccine strains are chosen

The World Health Organization and its Global Influenza Surveillance and Response System, GISRS, track circulating strains and recommend vaccine composition twice a year. Recommendations for the Northern Hemisphere are issued in February, and for the Southern Hemisphere in September, ahead of the next season.

What vaccine types exist

Most vaccines are inactivated, delivered as a shot. Some regions also offer a live attenuated nasal spray for eligible age groups. Cell-based and recombinant vaccines are available in some countries. Your local program guides which options you can get.

When to get vaccinated

Get vaccinated before flu begins circulating where you live. In temperate areas, that usually means early autumn. In tropical regions, flu can occur year-round with irregular peaks, so follow local health advice on timing.

Antiviral treatment

Antivirals can shorten illness and lower the risk of complications, especially when started within 48 hours of symptoms. Options include oseltamivir, zanamivir, peramivir, and baloxavir, depending on age and health status. In one randomized trial, baloxavir worked better than oseltamivir for confirmed influenza B in adolescents and adults. Treatment decisions depend on clinical judgment and local guidance.

Who should consider antivirals right away

People at high risk of complications
Anyone hospitalized with suspected or confirmed flu
People with severe or progressive disease

Do not wait for test results if high risk and flu is suspected. Start treatment as soon as possible after the first symptoms.

Flu vs. a cold vs. COVID-19

Flu and the common cold share many symptoms. Flu tends to start abruptly with fever and aches. COVID-19 can look similar to flu. Testing helps confirm the cause, which guides treatment and isolation advice. Follow current local testing guidance.

Proven prevention habits

Get your annual flu shot.
Stay home when sick.
Cover coughs and sneezes.
Wash hands with soap and water or use sanitizer.
Improve indoor ventilation.
Consider a well-fitting mask during peaks or if you are high risk.

These steps lower transmission in homes, schools, and workplaces.

Quick checklist

Task	Why it helps	When to act
Book your flu shot	Cuts risk of severe disease	Before local season starts
Check eligibility for nasal spray	Option for some age groups	Ask your clinician
Have a plan for antivirals	Faster care if you get sick	If symptoms start within 48 hours
Stock fever reducers and masks	Supports care at home, reduces spread	Before peak season
Update workplace or school sick policy	Limits outbreaks	At season start

Sources support these actions for both temperate and tropical settings.

What to do if you get sick

Rest, hydrate, and use fever reducers as advised. Seek care if you are high risk, have severe symptoms, or do not improve after a few days. A clinician can decide on antivirals and testing. Isolate from others while you are febrile and within the first few days of illness, when you are most contagious.

Common mistakes

Skipping the vaccine because you “never get sick.” Immunity and exposure change each year.
Waiting too long to start antivirals if you are high risk.
Going to work or school while sick.
Relying only on antibiotics. They do not treat viruses.

Public health agencies stress early vaccination, fast treatment for high-risk people, and staying home when ill.

Why it matters

Flu returns every year and can strain clinics and hospitals, especially when vaccination rates are low. Vaccination and early care reduce severe outcomes for families and communities. The approach is simple. Get vaccinated, practice hygiene, and seek care early if you are high risk.