Flu (Seasonal)

Influenza

In this factsheet:
• The Facts on Influenza
• Causes of Influenza
• Symptoms and Complications of Influenza
• Diagnosing Influenza
• Treating and Preventing Influenza

The Facts on Influenza

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, 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 4,000 to 8,000 people in Canada die each year from complications of influenza, and about 20,000 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 airborne 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.

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 his or her 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. Type B mainly affects humans and causes a milder disease, and it changes very little from year to year.

Type A influenza poses the most serious problems for humans. Strains of this type have also been found in birds, humans, 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.

There are infinite new varieties of type A influenza. Avian flu is a type of influenza A that had been seen only in birds, including chickens and ducks, until 1997 when the first human case was reported in Hong Kong. Although the avian flu affected mostly animals, it did cause a few cases of severe disease with a high risk of death in humans. Many birds were slaughtered in Hong Kong to prevent the disease from spreading. In early 2004, outbreaks of the avian flu re-emerged in Japan, South Korea, Vietnam, Thailand, Cambodia, China, and Indonesia. Human cases have been reported in Vietnam and Thailand. Millions of chickens have been affected and killed in the countries listed in an attempt to stop the spread of the disease.

Swine flu is another type of influenza A that is normally found in pigs. Swine flu is passed from pig to pig, and although it usually only infects pigs, there have been periodic infections in humans, resulting in H1N1 flu virus (human swine flu). Most of these cases occur in people with direct exposure to pigs (e.g., people working on pig farms). Person-to-person transmission of the H1N1 flu virus (human swine flu) does occur, but it is not clear how easily the virus is spread among people.

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 6 days, and children 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:

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.

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.