Influenza (flu)

What is it?

The CDC defines influenza as “a contagious respiratory illness caused by influenza viruses that infect the nose, throat and sometimes the lungs”, ranging from mild to severe and sometimes fatal. The WHO describes it as an acute respiratory infection that spreads easily by droplets, and most people recover without intervention.

Virus types according to the WHO: types A and B circulate seasonally (A is classified by surface proteins, and A(H1N1)pdm09 and A(H3N2) currently circulate; B has the Yamagata and Victoria lineages), type C is rare and causes mild infection, and type D affects only cattle. Winter epidemics occur in temperate regions, and in tropical regions circulation continues year-round with irregular outbreaks.

Figures according to the WHO: about a billion cases a year, including 3 to 5 million severe, and 290,000 to 650,000 respiratory deaths a year; and 99% of deaths in children under 5 from influenza-associated lower respiratory tract infections are in developing countries.

Causes

It is caused by influenza viruses. The CDC says most experts believe it spreads mainly by droplets produced when an infected person coughs, sneezes or talks, and less commonly by touching a contaminated surface and then the face. The incubation period is about two days (1 to 4). Infectiousness peaks in the first 3 days, and a healthy adult is contagious from a day before symptoms until 5 to 7 days after, and longer in young children and people with weak immunity.

Risk factors

According to the CDC and MedlinePlus, groups more likely to have complications: adults 65 and over, pregnant women, children under 5 (especially under 2), people with chronic illnesses such as asthma, diabetes and heart disease, and those with a body mass index of 40 or more. The WHO adds people with weak immunity and health workers, and the NHS adds lung, liver, kidney, brain and nerve diseases.

This page does not include the CDC's detailed list of groups more likely to have complications.

Symptoms

Symptoms usually begin suddenly (CDC): fever or feeling feverish and chills, cough, sore throat, runny or blocked nose, muscle or body aches, headache and tiredness. Vomiting and diarrhoea are more common in children. Not everyone affected has a fever; the CDC states that about 8% of positive cases had no symptoms.

The WHO states that symptoms begin 1 to 4 days after infection and usually pass within a week, though a cough may last two weeks or more. MedlinePlus and the NHS compare influenza with the common cold: a cold develops gradually and rarely comes with fever, while influenza starts suddenly with fever and severe exhaustion that prevents usual activity.

How is it diagnosed?

The WHO states that diagnosis is clinical during epidemics, and laboratory confirmation (antigen detection, virus isolation or RT-PCR) is required for definitive identification. According to MedlinePlus the care provider takes the history and symptoms and a swab from the nose or throat; rapid tests give a result in 15 to 20 minutes but are less accurate than the more comprehensive tests that take an hour or several hours.

Treatment

The treatment categories usually used are listed here; this is not a prescription. The doctor chooses the plan according to each person's condition; do not change any medicine or dose without a specialist.

“Most people with influenza recover on their own without medical care” (MedlinePlus). Supportive care: rest, fluids and relieving symptoms (WHO); the NHS lists rest, drinking water and fever-reducing painkillers such as paracetamol or ibuprofen, and that antibiotics do not help in viral infection.

Antivirals (prescribed with the doctor): a prescription medicine available as tablets, liquid, inhaled powder or intravenous, and it is not an antibiotic. Most people with mild influenza do not need it, and it is recommended for those more likely to have complications or who are severely ill or hospitalised. It is best within two days of symptom onset and remains useful if started later in high-risk or severely ill people; it shortens illness by about a day and may prevent some complications such as pneumonia (CDC). A medicine named on the CDC page is oseltamivir. The WHO recommends them for high-risk people or severe cases.

The CDC advises staying at home and avoiding contact except for medical care, and returning to activity when symptoms improve and the fever has gone for 24 hours without a fever-reducing medicine. Do not change a medicine or start another without consulting.

Complications

According to the CDC: bacterial pneumonia, ear infection, sinus infection, and worsening of chronic illnesses such as congestive heart failure, asthma and diabetes. MedlinePlus adds bronchitis, myocarditis (inflammation of the heart muscle), encephalitis (the brain) and myositis and rhabdomyolysis (the muscles). The WHO lists pneumonia and sepsis, and that severe illness is more common in at-risk groups.

Prevention

“The best way to prevent influenza is an annual vaccine” (MedlinePlus, CDC, WHO). The CDC and WHO list healthy habits: covering coughs and sneezes and disposing of tissues, washing hands regularly, avoiding contact with sick people and improving air quality. The NHS recommends vaccination for at-risk groups. This page does not cover vaccine details and effectiveness.

Living with it

During illness: rest, fluids, relieving symptoms and staying away from others until the fever has gone for 24 hours (CDC). See the doctor early if you are in the groups more likely to have complications because the antiviral works best early. A cough can last two weeks or more according to the WHO.

Common questions