Influenza

What is influenza?

Influenza, commonly called the flu, is a contagious respiratory illness caused by influenza viruses. These viruses infect the nose, throat, and sometimes the lungs. The question “what is influenza” often comes up because the word “flu” is used loosely in everyday speech for many illnesses, including stomach upsets. In medical terms, influenza refers specifically to infection with influenza viruses, and it is different from the common cold, which is usually caused by other viruses and tends to be milder.

Influenza can affect anyone, from infants to older adults. Most healthy people recover within one to two weeks without lasting problems. However, influenza can lead to serious complications in certain groups, including adults aged 65 and older, young children, pregnant women, and people with chronic health conditions such as asthma, diabetes, heart disease, or a weakened immune system. In some cases, influenza can lead to pneumonia (a lung infection), hospitalization, or, rarely, death.

Influenza spreads most actively during colder months in temperate climates, a period often called “flu season.” In many countries this runs roughly from autumn through early spring, though the exact timing varies from year to year and by region. The viruses change slightly over time, which is why people can catch influenza more than once in their lives and why vaccines are updated regularly.

Symptoms of influenza

Influenza symptoms usually begin suddenly, often within one to four days after exposure to the virus. This abrupt onset is one of the features that helps distinguish influenza from a common cold, which typically develops more gradually. Common influenza symptoms include:

• Fever (a raised body temperature) or feeling feverish and having chills — though not everyone with influenza develops a fever
• Cough, which is usually dry at first
• Sore throat
• Runny or stuffy nose
• Muscle or body aches, which can be severe
• Headache
• Fatigue (deep tiredness), which can be intense and may last longer than other symptoms
• Vomiting and diarrhea, which occur in some people, more often in children than in adults

Symptoms often follow a rough pattern over time. In the first two to three days, fever, chills, body aches, and headache tend to be most prominent. As the fever settles, respiratory symptoms such as cough and nasal congestion may become more noticeable. The cough and a general feeling of tiredness or weakness can persist for a week or more after the fever has resolved. In many cases, feeling completely back to normal takes one to two weeks, and sometimes longer in older adults.

Symptoms can also differ by age group. Older adults may have less obvious fever but more confusion or weakness. Young children may have higher fevers, irritability, and more stomach symptoms. People with weakened immune systems may have symptoms that are milder at first but last longer or worsen unexpectedly.

It is worth noting that influenza symptoms overlap with those of other respiratory infections, including COVID-19 and respiratory syncytial virus (RSV). Because the symptoms alone often cannot tell these illnesses apart, testing may be needed when the diagnosis matters for treatment decisions.

Causes and risk factors

Influenza causes are viral. The illness is caused by influenza viruses, which are grouped into types. Influenza A and influenza B viruses are responsible for the seasonal epidemics that affect people each year. Influenza A viruses are further divided into subtypes based on proteins on their surface, and influenza B viruses are divided into lineages. The specific code J11.1 refers to influenza with other respiratory manifestations where the exact virus has not been laboratory-confirmed — in other words, an illness that looks like influenza clinically but has not been identified by a test.

The virus spreads mainly through droplets released when an infected person coughs, sneezes, or talks. These droplets can land in the mouths or noses of people nearby. Less often, a person may become infected by touching a surface contaminated with the virus and then touching their own mouth, nose, or eyes. People with influenza are usually most contagious in the first few days of illness, and many can spread the virus starting about a day before symptoms appear.

Anyone can catch influenza, but certain factors raise the risk of infection or of severe illness:

• Age: children under five (especially under two) and adults aged 65 and older are at higher risk of complications.
• Pregnancy: pregnant women and women who have recently given birth face a higher risk of severe illness.
• Chronic medical conditions: asthma, chronic obstructive pulmonary disease (a long-term lung disease), heart disease, diabetes, kidney disease, liver disease, and neurological conditions can all increase risk.
• Weakened immune system: from conditions such as HIV, cancer treatment, or medications that suppress immunity.
• Obesity: a very high body weight has been associated with more severe influenza illness.
• Living or working environment: crowded settings such as nursing homes, schools, and dormitories make transmission easier.

Annual vaccination is the most reliable way to reduce the risk of catching influenza and of developing severe illness if infection occurs. Because the circulating viruses change from season to season, vaccines are reformulated each year, which is why yearly vaccination is generally recommended, particularly for people in higher-risk groups. Good hand hygiene, covering coughs and sneezes, and staying home when ill also help limit spread.

Diagnosis

Influenza diagnosis often begins with a conversation and a physical examination. During flu season, when influenza is known to be circulating in the community, a doctor may make a clinical diagnosis based on typical symptoms — sudden fever, cough, body aches, and fatigue — without ordering any tests. This is often sufficient for otherwise healthy people with mild illness.

When confirming the diagnosis matters — for example, in hospitalized patients, people at high risk of complications, or when treatment decisions depend on the result — doctors can use laboratory tests. Common options include:

• Rapid influenza diagnostic tests (RIDTs): these detect parts of the virus from a swab taken from the nose or throat and give results within about 10 to 15 minutes. They are convenient but can miss some infections, meaning a negative result does not always rule out influenza.
• Molecular tests (such as PCR): polymerase chain reaction tests detect the genetic material of the virus. They are more accurate than rapid antigen tests and can often distinguish influenza A from influenza B, and influenza from other viruses such as the virus that causes COVID-19. Results may take from under an hour to a day or more depending on the laboratory.
• Viral culture: growing the virus in a laboratory is used mainly for public health surveillance rather than routine patient care, because it takes days.

Imaging is not needed to diagnose influenza itself. However, if a doctor suspects a complication such as pneumonia — for example, because of chest pain, breathing difficulty, or symptoms that worsen after initial improvement — a chest X-ray may be ordered to look at the lungs. Blood tests may be used in hospitalized patients to assess overall condition, but they do not confirm influenza on their own.

Influenza is typically managed by family physicians or general practitioners, and complicated or severe cases may be referred to specialists such as those in an infectious diseases department, which handles illnesses caused by viruses, bacteria, and other microorganisms.

Treatment options

Influenza treatment depends on how severe the illness is and how likely a person is to develop complications. For most otherwise healthy people, the illness resolves on its own and treatment focuses on comfort while the body clears the virus. An overview of how the condition is managed can also be found on the influenza treatment page.

Supportive care and watchful waiting

For mild to moderate illness in healthy people, doctors often recommend supportive care rather than specific medication. This typically includes:

• Rest: allowing the body to direct energy toward recovery.
• Fluids: drinking enough water, broth, or other fluids to prevent dehydration, particularly if there is fever, vomiting, or diarrhea.
• Fever and pain relief: over-the-counter medicines such as acetaminophen (paracetamol) or ibuprofen can reduce fever, aches, and headache. Aspirin should not be given to children or teenagers with influenza because of the risk of Reye’s syndrome, a rare but serious condition affecting the liver and brain.
• Staying home: to recover and to avoid spreading the virus to others, usually until at least 24 hours after fever has resolved without fever-reducing medicine.

Antibiotics do not work against influenza, because antibiotics treat bacterial infections and influenza is caused by a virus. A doctor may prescribe antibiotics only if a bacterial complication, such as bacterial pneumonia or a bacterial ear or sinus infection, develops on top of the viral illness.

Antiviral medications

Antiviral drugs are medicines that act specifically against viruses. For influenza, options include oseltamivir (taken by mouth), zanamivir (inhaled), peramivir (given intravenously), and baloxavir (a single oral dose), depending on availability and the individual patient. These medicines can shorten the duration of illness, often by about a day, and may reduce the risk of complications, particularly when started early — ideally within 48 hours of symptom onset.

Antivirals are not necessary for everyone. Doctors usually consider them for people who are hospitalized, who have severe or worsening illness, or who are at higher risk of complications, such as older adults, young children, pregnant women, and people with chronic conditions. Your doctor may still consider antiviral treatment after the 48-hour window in severe or high-risk cases. Like all medicines, antivirals can have side effects, such as nausea or vomiting, which your doctor can discuss with you.

Hospital care

Most people with influenza do not need hospital treatment. Hospitalization may be required when complications develop, such as pneumonia, dehydration, worsening of an existing chronic condition, or difficulty breathing. In the hospital, care may include intravenous fluids, oxygen therapy, antiviral medication, treatment of any bacterial infection, and, in severe cases, support in an intensive care unit. There is no surgery or procedure that treats influenza itself; procedures are only relevant if a rare complication requires them. At hospital groups such as Acibadem, complicated influenza cases are generally managed within infectious diseases and pulmonology (lung medicine) services.

Living with influenza and outlook

For the great majority of healthy people, the outlook after influenza is good. Fever and the most intense symptoms usually settle within about three to five days, and most people feel largely recovered within one to two weeks. Fatigue and a lingering cough can last longer — sometimes several weeks — and this is common rather than a sign that something has gone wrong, provided symptoms are gradually improving.

During recovery, it helps to rest, eat when able, drink fluids regularly, and return to normal activities gradually. Pushing too hard too soon can prolong tiredness. Smokers may notice a longer cough, and reducing or stopping smoking supports lung recovery.

The outlook is less predictable for people in higher-risk groups. Complications such as pneumonia, worsening of heart or lung disease, sinus and ear infections, and, rarely, inflammation of the heart, brain, or muscles can occur. Recovery after complicated influenza can take considerably longer, and some people, particularly frail older adults, may take weeks or months to regain their previous strength. No honest outlook statement can offer guarantees; what can be said is that early attention to warning signs, appropriate treatment, and annual vaccination together improve the chances of a smooth course.

Having influenza once does not protect against all future infections, because the viruses change over time. Yearly vaccination remains the main preventive step, alongside hand hygiene and avoiding close contact with people who are ill.

Frequently asked questions

What is influenza and how is it different from a cold?

Influenza is a respiratory illness caused by influenza viruses. It differs from the common cold mainly in how it starts and how it feels: influenza typically begins suddenly with fever, chills, marked body aches, and exhaustion, while a cold develops gradually and usually causes milder symptoms centered on the nose and throat. Because symptoms overlap, only a laboratory test can distinguish the two with certainty.

How long does influenza last?

In many cases, fever and the most intense symptoms last three to five days, and most people feel substantially better within one to two weeks. Cough and tiredness can linger for several weeks. Recovery may take longer in older adults, young children, and people with chronic health conditions. If symptoms are worsening rather than improving after the first week, it is sensible to seek medical advice.

Can influenza heal on its own without treatment?

Yes, in most otherwise healthy people influenza resolves on its own with rest, fluids, and simple fever-reducing medicines. Antiviral medication is not needed for everyone. However, people at higher risk of complications — such as older adults, pregnant women, and those with chronic illnesses — should consider contacting a doctor early, because antiviral treatment works best when started within the first 48 hours of symptoms.

How serious is influenza?

For most healthy people, influenza is an unpleasant but self-limiting illness. For some, it can be serious: complications such as pneumonia, dehydration, and worsening of existing heart or lung conditions can lead to hospitalization and, rarely, death. The risk of severe illness is highest in adults aged 65 and older, very young children, pregnant women, and people with chronic diseases or weakened immune systems.

How is influenza diagnosed?

Doctors often diagnose influenza clinically during flu season, based on typical symptoms such as sudden fever, cough, and body aches. When confirmation matters, a swab from the nose or throat can be tested with a rapid antigen test or a more accurate molecular (PCR) test. Imaging such as a chest X-ray is used only if a complication like pneumonia is suspected.

Do antibiotics work for influenza?

No. Antibiotics kill bacteria, and influenza is caused by a virus, so antibiotics have no effect on the flu itself. Taking antibiotics unnecessarily can cause side effects and contributes to antibiotic resistance. A doctor may prescribe antibiotics only if a bacterial infection, such as bacterial pneumonia or an ear infection, develops as a complication of influenza.

How can I avoid catching or spreading influenza?

Annual vaccination is the most effective preventive measure, particularly for people at higher risk. Beyond vaccination, washing hands frequently, avoiding touching the face, covering coughs and sneezes, keeping distance from people who are ill, and staying home when sick all help limit the spread of the virus. People with influenza are usually advised to stay home until at least 24 hours after their fever has gone without fever-reducing medicine.

When to see a doctor

Most cases of influenza can be managed at home. However, some symptoms suggest a complication or severe illness and warrant urgent medical attention. Seek prompt care — including emergency care where appropriate — if you or someone you are caring for develops any of the following:

• Difficulty breathing or shortness of breath, at rest or with minimal activity
• Persistent chest pain or pressure, or pain or pressure in the abdomen
• Bluish or gray lips, face, or fingertips
• Confusion, unusual drowsiness, or difficulty waking
• Seizures
• Signs of dehydration, such as not urinating, dizziness when standing, or, in infants, no wet diapers for eight hours and crying without tears
• Symptoms that improve and then return or worsen, especially a returning fever with a worsening cough — this can signal pneumonia
• Severe muscle pain or marked weakness that makes walking difficult
• In children: fast or labored breathing, ribs pulling in with each breath, refusal to drink, a fever with a rash, or being unusually unresponsive or irritable

People at higher risk of complications — including adults over 65, pregnant women, young children, and anyone with a chronic illness or weakened immune system — should consider contacting a doctor early in the course of influenza-like symptoms, even without red-flag signs, because early antiviral treatment is most effective in the first 48 hours. When in doubt about how serious symptoms are, it is safer to seek medical advice than to wait.