The flu, caused by an influenza virus, is relatively common. The Centers for Disease Control and Prevention (CDC) reports that the seasonal flu affects up to 20 percent of Americans each year. Many people can fight symptoms with plenty of rest and fluids. However, certain high-risk groups may have dangerous and even life-threatening complications.

The CDC estimates that between 3,000 and 49,000 people in the United States die each year from the flu. The World Health Organization estimates that, globally, between a quarter-million and a half-million people die from flu complications each year.

A CDC study looking at data from 1979 to 2001 found that an average of 200,000 Americans needed hospital care each year for respiratory and heart complications from the flu virus. The study also found that flu-related hospital visits increased with each year. Hospital visits reached a high of 430,960 in 1997–1998.

Certain groups are at higher risk of the flu. According to the CDC, these groups should receive first priority when there is a shortage of flu vaccine. Risk factors include age, ethnicity, existing conditions, and other factors.

Age groups who have increased risk include:

  • children younger than five years
  • children and teens younger than 19 years who receive aspirin therapy
  • people who are 65 years or older

Ethnic groups who have higher risk include:

  • Native Americans
  • Alaskan Natives

People with any of the following conditions are also at higher risk of flu complications:

  • asthma
  • heart and lung conditions
  • chronic endocrine disorders, such as diabetes mellitus
  • chronic health conditions affecting the kidneys and liver
  • chronic neurological and neurodevelopmental disorders, such as epilepsy, stroke, and cerebral palsy
  • chronic blood disorders, such as sickle cell anemia
  • chronic metabolic disorders

Other people who are at increased risk include:

  • people with weakened immune systems, either due to disease (such as cancer, HIV, or AIDS) or long-term steroid medication use
  • women who are pregnant
  • morbidly obese people with a body mass index (BMI) of 40 or higher

These groups should monitor their flu symptoms closely. They should also seek immediate medical care at the first sign of complications. These often appear just as main flu symptoms like fever and fatigue start to go away.

Older adults

People who are 65 years or older are at the greatest risk of complications and death from the flu. The CDC estimates that these people make up 54 to 70 percent of flu-related hospital visits. They also account for 71 to 85 percent of flu-related deaths, which is why it’s so important for older adults to receive a flu shot.

The U.S. Food and Drug Administration (FDA) has approved Fluzone High-Dose, a higher-dose vaccine, for people who are 65 years or older. Fluzone High-Dose contains four times the amount of antigens as the normal flu vaccine. Antigens stimulate the immune system to produce antibodies, which fight the flu virus.

Another flu vaccine option for older adults is called FLUAD. It contains a substance for stimulating a stronger immune response.

Pneumonia is an infection of the lungs that causes the alveoli to become inflamed. This causes symptoms such as cough, fever, shaking, and chills. Pneumonia can develop and become a serious complication of the flu. It can be especially dangerous and even deadly for people in high-risk groups.

Seek medical treatment immediately if you have any of the following symptoms:

  • severe cough with large amounts of mucus
  • trouble breathing
  • shortness of breath
  • severe chills or sweating
  • fever higher than 102°F (38.9°C) that’s not going away, especially if you also have chills or sweating
  • chest pains

Pneumonia is highly treatable, often with simple home remedies such as sleep and plenty of warm fluids. However, smokers, older adults, and people with heart or lung problems are especially prone to pneumonia-related complications. Pneumonia-related complications include:

Learn more: Pneumonia »

This complication is caused by irritation of the mucous membranes of the bronchi in the lungs.

Symptoms of bronchitis include:

  • cough (often with mucus)
  • chest tightness
  • fatigue
  • mild fever
  • chills

Most often, simple remedies are all that’s needed to treat bronchitis. These include:

  • resting
  • drinking plenty of fluids
  • using a humidifier
  • taking over-the-counter pain medications

You should contact your doctor, though, if you have a cough with a fever greater than 100.4°F (38°C). You should also call if your cough does any of the following:

  • lasts longer than three weeks
  • interrupts your sleep
  • produces mucus of a strange color
  • produces blood

Untreated, chronic bronchitis can lead to more serious conditions, including pneumonia, emphysema, heart failure, and pulmonary hypertension.

Read more: Understanding chronic bronchitis »

Sinusitis is the swelling of the sinuses. Symptoms include:

  • nasal congestion
  • sore throat
  • postnasal drip
  • pain in the sinuses, upper jaw, and teeth
  • a reduced sense of smell or taste
  • cough

Sinusitis can often be treated with over-the-counter saline spray, decongestants, and pain relievers. Your doctor may also prescribe a nasal corticosteroid like fluticasone (Flonase) or mometasone (Nasonex) to reduce inflammation.

Symptoms that call for immediate medical attention include:

  • pain or swelling near the eyes
  • swollen forehead
  • severe headache
  • mental confusion
  • vision changes, such as seeing double
  • difficulty breathing
  • neck stiffness

These may be signs of sinusitis that has worsened or spread.

Better known as an ear infection, otitis media causes inflammation and swelling of the middle ear. Symptoms include:

  • chills
  • fever
  • hearing loss
  • ear drainage
  • vomiting
  • mood changes

An adult with ear pain or discharge should see their doctor as soon as possible. A child should be taken to their doctor if:

  • symptoms last longer than a day
  • ear pain is extreme
  • ear discharge appears
  • they are not sleeping
  • they are moodier than usual

This rare condition occurs when a flu virus enters the brain tissue and causes inflammation of the brain. This can lead to destroyed nerve cells, bleeding in the brain, and brain damage.

Symptoms include:

  • severe headache
  • high fever
  • vomiting
  • light sensitivity
  • drowsiness
  • clumsiness

Though rare, this condition may also cause tremors and difficulty with movement.

Seek immediate medical care if you have any of the following symptoms:

  • severe headache or fever
  • mental confusion
  • hallucinations
  • severe mood changes
  • seizures
  • paralysis
  • double vision
  • speech or hearing problems

Symptoms of encephalitis in young children include:

  • protrusions in the soft spots on an infant’s skull
  • body stiffness
  • uncontrollable crying
  • crying that gets worse when the child is picked up
  • loss of appetite
  • nausea and vomiting

Learn more: Encephalitis »

Most flu symptoms resolve within one to two weeks. If your flu symptoms worsen or do not subside after two weeks, contact your doctor.

A yearly flu vaccine is the best preventive measure for people at high risk of flu-related complications. Good hygiene, regular hand-washing, and avoiding or limiting contact with infected people can also help prevent the spread of the flu.

Early treatment is also key to successful treatment of complications. Most of the complications mentioned respond well to treatment. That said, many can become more serious without proper treatment.