Influenza
(Flu)
In this condition factsheet:
The Facts on Influenza
The flu is a respiratory (i.e., nose, throat, and lung) infection that can be caused by 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 season typically starts between October and December, and usually ends around February – however, it can sometimes last until May. Up to 25% of the population may be infected in an average year. Stronger epidemics (e.g., when more people than expected get the flu in a given area or season) come every 2 or 3 years, infecting about 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 people in Canada die each year from complications of influenza, and about 12,000 people with the flu are hospitalized. Most of these people have other medical conditions, are seniors, or are very young children.
Causes of Influenza
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 that are sent through the air when someone infected with the virus coughs or sneezes. Other people inhale the airborne virus and can become infected.
The flu virus can also be spread when someone touches a surface (e.g., doorknobs, countertops, telephones) that has the virus on it and then touches his or her nose, mouth, or eyes. The flu is most easily spread in crowded places such as schools and offices.
There are 3 types of influenza viruses that can infect humans: 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, largely affecting children. Type B mainly affects humans, and usually causes a milder illness. It changes very little from year to year.
Type A influenza poses the most serious problems for humans, and is responsible for the majority of flu epidemics and pandemics. Strains of this type have also been found in birds, horses, pigs, seals, whales, and ferrets. Viruses that affect 2 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 become contagious before symptoms appear, often just the day after the virus enters the body. Adults remain infectious (i.e., they can spread the virus to others) for about 5 to 7 days, and young children may remain infectious for up to 10 days.
Symptoms and Complications of Influenza
Initial flu symptoms include headaches, chills, and a cough. Symptoms such as fever, loss of appetite, and muscle aches are soon to 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 |
Fever |
Uncommon |
Usually present, high (38°C to 40°C or 100°F to 104°F); lasts 3 to 4 days |
Headache |
Uncommon |
Very common |
Aches and pains |
Slight |
Common and often severe |
Fatigue and weakness |
Mild |
Can last 2 to 3 weeks |
Extreme exhaustion |
Never |
Very common at the start |
Stuffy nose |
Common |
Sometimes |
Sneezing |
Common |
Sometimes |
Sore throat |
Common |
Sometimes |
Chest discomfort and cough |
Mild to moderate, hacking cough |
Common |
For most people, the flu lasts 1 to 2 weeks, but it can last for up to a month. The main complications are secondary bacterial infections of the sinuses or lungs (pneumonia). Symptoms include fever, chills, body aches, cough, and sputum production (also called phlegm; usually brown, green, or yellow in colour). 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 people in the second or third trimester of pregnancy are at an increased risk of complications from the flu.
Knowing how to recognize the signs of flu sickness that require immediate attention is important. Here is a simple list of key emergency warning signs to watch for:
Children:
- 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 worsened cough
Adults:
- 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 worsened cough
