What is Influenza (Flu)? Causes, Symptoms, Diagnosis, Treatment, and Complications

Influenza is a very contagious, acute respiratory infection caused by the influenza virus. We commonly call it the flu or simply flu. People of all ages can become ill with the virus. However, it is most dangerous for people with weakened immune systems, pregnant women, seniors, and young children.

According to the World Health Organization:

“Hospitalization and death occur mainly among high-risk groups. Worldwide, these annual epidemics are estimated to result in about 3 to 5 million cases of severe illness, and about 290 000 to 650 000 respiratory deaths.”

“Research estimates that 99% of deaths in children under 5 years of age with influenza related lower respiratory tract infections are found in developing countries ”

Causes of influenza

There are three types of influenza viruses – A, B, and C. A and B are responsible for seasonal epidemics, while C typically causes mild respiratory illness.

The virus spreads in the air in droplets when an infected individual sneezes, coughs, or simply talks. You can become infected by touching a contaminated surface, i.e., a surface that a person with flu has touched.

Signs and symptoms

A sign, such as a skin rash or swollen ankle, is something the patient and also doctors, family members, and friends can detect. However, only the patient can feel, sense, or detect a symptom. Examples of symptoms are headaches, suicidal thoughts, and blurred vision.

Influenza signs and symptoms can range from very mild to severe, and include:

  • Body aches.
  • Chills.
  • Cough.
  • Fatigue.
  • Fever.
  • Headache.
  • Runny or stuffy nose.
  • Sore throat.
  • Children may experience diarrhea and vomiting (rare in adults).

Signs and symptoms usually appear up to four days after exposure to the virus.


Doctors usually diagnose influenza based on the patient’s signs and symptoms. A laboratory test can confirm whether a flu virus is present. An RIDT (rapid influenza diagnostic test) involves collecting a sample of throat, nose, and nasopharyngeal secretions.

Treatment for influenza

There is no treatment for flu. However, some antiviral drugs may help reduce the duration of the illness, prevent complications, and relieve symptoms. They are most effective if you take them within 48 hours of symptom onset.

If you have flu, you should rest and consume plenty of fluids. Some OTC medications can help treat fever and pain. OTC stands for over the counter. OTC drugs are those that you can get without a doctor’s prescription.

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Complications of influenza can be serious and even life-threatening, especially for high-risk groups. These are the most common complications:

  • Bronchitis.
  • Dehydration.
  • Ear infections.
  • Pneumonia.
  • Sinus infections.
  • Chronic illnesses or medical conditions get worse.

Regarding some of the complications, the US CDC says:

“Other possible serious complications triggered by flu can include inflammation of the heart (myocarditis), brain (encephalitis) or muscle tissues (myositis, rhabdomyolysis), and multi-organ failure (for example, respiratory and kidney failure).”

Prevention of influenza

  • If you take a flu shot (vaccination) every year, you have a significantly lower chance of developing the infection. Doctors across the world recommend the flu vaccine for everybody aged more than six months, especially for those at high risk of complications.
  • Wash your hands regularly with soap and water (warm water if possible).
  • Whenever you sneeze or cough, cover your nose and mouth with a tissue or handkerchief. If you have no tissue, do so into the inner part of your elbow.
  • Avoid close contact with people who are sick with flu.
  • When you are sick, don’t go out.
  • Disinfect all surfaces that may be contaminated with the influenza virus.

When to call your doctor

At risk people who develop the flu should contact their doctor as soon as the signs and symptoms appear.

Regarding otherwise healthy individuals, ucsfhealth.org has the following advice:

“If you are otherwise healthy and not at increased risk of complications, seek medical advice if your flu symptoms are unusually severe.”

Unusually severe symptoms may include breathing difficulties, a severe sore throat, coughing up a lot of yellow or green mucus, or feeling faint.

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