Medical Conditions - Influenza

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The Facts

The flu is a respiratory (i.e., nose, throat, and lung) infection that can be caused by a variety of influenza viruses. Many people use the word "flu" when they actually have a cold. Although the common cold is also caused by viruses, the flu and common cold differ in several ways.

In North America, the flu almost always strikes between November and April. Up to 25% of the population may be infected in an average year. Stronger epidemics (i.e., when the flu occurs in more people than expected in a given area or season) come every 2 or 3 years, infecting twice as many people as during an "off" year.

Most people who get the flu will recover within 1 to 2 weeks, but some people are at risk of developing complications such as pneumonia. On average, about 3,500 Canadians die each year from complications of influenza, and  over 12,000 are hospitalized. Most of these people have other medical conditions, are seniors, or are very young children.


Influenza is contagious, which means it can be spread easily from person to person. Viruses that cause influenza spread from person to person mainly by droplets of respiratory fluids sent through the air when someone infected with the virus coughs or sneezes. Other people inhale the airborne virus and can become infected.

Flu virus can live on some surfaces for up to 24 hours. This means that, in some cases, the flu can be spread when someone touches a surface (e.g., doorknobs, countertops, telephones) that has the virus on it and then touches their nose, mouth, or eyes. The flu is most easily spread in crowded places such as schools and offices.

There are three families of influenza virus: A, B, and C. Type C more commonly affects ducks, geese, turkeys, and chickens, but it has also been involved in a small percentage of human cases, most of which are in children. Type B mainly affects humans and usually causes a milder disease, and it changes very little from year to year.

Type A influenza poses the most serious problems for humans and causes 95% of cases. Strains of this type have also been found in birds, horses, pigs, seals, whales, and ferrets. Viruses that affect two different species sometimes combine and mix-and-match genetic information to create a new strain that nobody is immune to and for which no vaccine has been prepared.

The flu takes 1 to 4 days to incubate in humans, but infected people can become contagious starting the day before symptoms appear. Adults remain infectious (i.e., they can spread the virus to others) for about 6 days, and children remain infectious for up to 10 days.

Symptoms and Complications

Initial flu symptoms include headaches, chills, and a cough. Symptoms such as fever, loss of appetite, and muscle aches soon follow. Other symptoms such as nausea, vomiting, and diarrhea are rare in adults but more common in children.

Since many people think they have the flu when it’s actually a bad cold, here’s a quick guide to help you tell the difference:

Symptom Cold Flu

Symptom onset





usually present, high (38°C to 41°C or 102°F to 104°F); lasts 3 to 4 days



very common

aches and pains


common and often severe

fatigue and weakness


significant; can last 2 to 3 weeks

extreme exhaustion


very common at the start

stuffy nose






sore throat



chest discomfort and cough

mild to moderate, hacking cough

common; often severe with painful cough

For most people, the flu lasts 1 or 2 weeks, but it can last for up to 1 month. The main complications are secondary bacterial infections of the sinuses or lungs (pneumonia). Symptoms include fever; chills; and yellow, green, or brown sputum (nasal discharge). Children are prone to ear infections like otitis media.

People in nursing homes are at a higher risk of complications from flu because they may have weak immune systems and often have other medical problems. People with asthma, chronic obstructive pulmonary disease, or congestive heart failure are also at a higher risk of developing bacterial infections like pneumonia. Additionally, people with diabetes and pregnant women are at an increased risk of complications from the flu.

In American studies, influenza hospitalization rates for children under 5 years of age were second only to the hospitalization rate in people over 65 years of age. School-aged children have the highest infection rates both during and between epidemics, and they are likely to be infected early in the season. Households with school-aged children have infection rates 30% above the average rate.

Knowing how to recognize emergency signs of flu sickness that require immediate attention is important. Here is a list of key emergency warning signs to watch for:


  • Fast breathing or trouble breathing
  • Bluish skin colour
  • Not drinking enough fluids
  • Not waking up or interacting
  • Fever with rash
  • Flu-like symptoms improving then returning with fever and worse cough


  • Difficulty breathing
  • Pain or pressure in chest or abdomen
  • Sudden dizziness
  • Confusion
  • Severe or persistent vomiting
  • Flu-like symptoms improving then returning with fever and worse cough

Making the Diagnosis

Since the symptoms of an influenza infection are different from the common cold, the diagnosis can be made fairly quickly. Your doctor will be able to tell that you have the flu if you have at least some or most of these symptoms:

  • aches and pains everywhere, especially in the back and legs
  • bad headache
  • burning sensation in the chest
  • dry cough at first, then bringing up sputum
  • high fever
  • nausea and possible vomiting
  • sore throat
  • runny or stuffy nose
  • extreme tiredness

If there is any doubt, your doctor can make sure that it’s the flu by taking a throat swab and testing it for the virus. This is rarely needed.

Treatment and Prevention

The normal treatment for flu is rest and plenty of liquids. Treatment also includes ways to prevent spreading the flu virus, such as proper handwashing, keeping common surfaces clean, and coughing or sneezing into your arm or sleeve.

Medications for specific symptoms can help. Cough suppressants can be used for cough. Ibuprofen or acetaminophen can be used to treat symptoms of the flu, such as aches and fever. Children and teenagers with flu shouldn’t take acetylsalicylic acid (ASA). The combination of influenza and ASA is linked to Reye’s syndrome, a rare but serious condition affecting the brain and liver. Many over-the-counter cold medications contain ASA or other salicylates. Ask your doctor or pharmacist about this.

Antibiotics are not effective against viral infections like flu and the cold, but they are prescribed for complications such as pneumonia or other bacterial infections.

Antiviral medications oseltamivir and zanamivir are sometimes used to treat the flu. These medications can help shorten the duration of the flu and reduce symptoms and work best if they are taken within 48 hours of the start of symptoms. Antiviral medications are also used to prevent flu infection for some people after they come into close contact with a person who has the flu, such as flu-infected people who live in the same household. Generally, antivirals are not recommended for most people; however, they may be recommended for people at risk for flu complications. Your doctor can decide whether you should start antiviral medications.

Flu antibodies can prevent the flu. The only way to generate antibodies is to be infected or to get vaccinated. Because the flu viruses can change from year to year, vaccination needs to be repeated every year. Keep in mind that all flu vaccines take 2 weeks to start providing protection, so the key is to get vaccinated early.

Each spring, a worldwide network of physicians and testing labs decide which flu strains are likely to cause trouble and design that year’s vaccine accordingly. The effectiveness of the vaccine can vary from season to season, which means there is still a chance that you can get the flu – although your symptoms may be milder. The flu vaccine should be given to anyone considered high risk for flu complications, those that can pass the flu to those at high risk (i.e. health care providers, caregivers, etc.), those that provide essential community services, and anyone who wants to avoid the flu.

High-risk groups for flu complications include:

  • anyone aged 65 years or older
  • young children under 5 years old (especially if they are less than 2 years old)
  • people with underlying medical conditions, including people with:
    • asthma and other chronic lung diseases (e.g., chronic obstructive pulmonary disease [COPD], cystic fibrosis)
    • diabetes and other metabolic diseases
    • heart disease (e.g., coronary artery disease, congestive heart failure, congenital heart disease)
    • kidney disease
    • a weakened immune system (immunocompromised), which can be caused by:
      • HIV/AIDS, an infection that attacks the immune system
      • cancer
      • medications that suppress the immune system
    • blood disorders (e.g., anemia, sickle cell anemia)
    • neurologic and neurodevelopmental disorders that affect their ability to swallow and breathe
    • morbid obesity (BMI of 40 or higher)
  • residents of nursing homes or other chronic care facilities, regardless of age
  • children and adolescents receiving long-term ASA therapy
  • pregnant women (especially if they are in their second or third trimester)
  • Indigenous peoples

People who should not receive a flu shot include children less than 6 months of age and those who have had a severe allergic reaction to a previous flu vaccine. Be sure to tell your health care provider about this and any other allergies you may have before you are given your flu shot.

You can reduce your risk of getting the flu by washing your hands regularly using soap and warm water or an alcohol-based hand sanitizer. Also, cough or sneeze into a tissue or into your sleeve. Dispose of the tissue right away. If you have flu symptoms, stay home from work or school and avoid contact with people who are at a high risk of flu complications (e.g., seniors, nursing home residents).

*All medications have both common (generic) and brand names. The brand name is what a specific manufacturer calls the product (e.g., Tylenol®). The common name is the medical name for the medication (e.g., acetaminophen). A medication may have many brand names, but only one common name. This article lists medications by their common names. For information on a given medication, check our Drug Information database. For more information on brand names, speak with your doctor or pharmacist.

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