Historically known as la grippe, influenza is an acute infectious epidemic disease caused by a virus. There are four main types of the virus A, B, C and D and many strains. The virus enters the body through the respiratory tract, but soon spreads to cause symptoms that include fever, chills, headache, sore throat, cough, gastrointestinal disturbances, muscular pain and neuralgia. Recovering from an attack provides temporary immunity, but only to the particular strain involved. Outbreaks of the disease typically demonstrate high morbidity but low mortality rates, usually killing only the very old and the very young (deaths in these cases are usually related to complications such as bacterial pneumonia). There is no cure for influenza. The best treatment is a combination of bed rest and increased fluid intake. Neither is there a sure means of prevention. Vaccines are available and are manufactured to provide immunization against the annually active influenza strains. The best prevention is frequent and thorough hand washing, staying away from others during illness and a healthy lifestyle.

It was only with the eradication, through advances in public health and medicine, of more major infectious diseases, that influenza was recognized as a serious threat. In Canada, influenza appeared in epidemic proportions on at least 7 occasions during the 19th century. The effects of the epidemic of 1832 were masked by cholera and those of the epidemic of 1847-48 by typhus, but the eradication of other diseases was not the only reason influenza grew in importance by the end of the 1800s. The epidemic of 1889-90 was particularly virulent, affecting 40% of the world's population.

Spanish Flu

The most damaging epidemic of influenza - for Canada and the world - appeared after World War I. The Spanish Flu of 1918-19 killed some 21 million people, including about 50 000 Canadians. It demonstrated a perverse tendency to kill the young and hearty. A long-term consequence for some victims was the development of a Parkinsonian syndrome, including a marked tremor. The Spanish Flu was brought into Canada by returning troops and made its way into even the remotest communities. Some entire villages were wiped out by the disease, and Labrador and Québec were particularly hard hit. Some areas unsuccessfully tried quarantine. All medical facilities and personnel were soon overtaxed and volunteers organized infirmaries in schools and hotels.

The epidemic brought not only death but social and economic disruption as well. Children were left parentless and many families found themselves without the chief wage earner. Armies on both sides of the war were temporarily debilitated. Businesses lost profits because of lack of demand for their products or because they were unable - as a result of a reduced work force - to meet the demand. Municipal governments, in an attempt to halt the spread of the disease, closed all except necessary services, and provinces enacted laws regarding quarantine and enforced the wearing of masks in public. Although the Canadian population unhappily accepted these restrictions, it defied the federal government's request that World War I-victory celebrations be postponed until December 1. The Spanish influenza strain, although decreasingly virulent, remained active in Canada until the mid-1920s. The establishment of the federal Department of Health in 1919 was a direct result of the epidemic. Pneumonia contracted by a patient weakened by influenza rather than influenza itself was the major cause of death, and pneumonia lost its impact with the discovery of penicillin, which Canada began to manufacture during World War II.

Today health facilities are much improved, knowledge of the virus has improved and the population generally is in better health and is more educated about personal hygiene and contagious disease. An estimated 10 to 25% of Canadians get the flu each year. Most of these patients recover completely, although there is a small percentage of the population with compromised immune systems who may develop complications such as pneumonia. Approximately 4000 to 8000 Canadians, many of them seniors, die of pneumonia caused by influenza annually in Canada.

Swine Flu

"Swine flu," or swine influenza, is an acute respiratory disease occurring in pigs caused by a subtype of the influenza A virus. There are 4 main swine flu subtypes: H1N1, H1N2, H3N1, and H3N2. Influenza viruses can mutate or combine with other types of flu viruses to create new, or novel, strains, like the type A H1N1 virus that has caused widespread outbreaks among humans, such as the well-known outbreak in 1976. Swine flu came to the world's attention again in the spring of 2009 during a widespread flu outbreak that became a pandemic. The new strain behaved differently than the normally species-specific virus, crossing the species barrier to cause disease in humans. It was also contagious from humans back to pigs, which occurred on an Alberta farm in spring 2009.

The 2009 outbreak of swine flu appeared to originate in Mexico; it was first identified there in March. Subsequent cases occurred in Canada and internationally, reaching more than 70 countries with more than 30 000 cases reported. The symptoms of the new virus tended to be mild and difficult to distinguish from seasonal influenza symptoms; most infected people recovered without medical treatment, but it was fatal for some with chronic conditions, especially respiratory conditions, or those who were generally in poor health.

The first case of the H1N1 swine flu in Canada in 2009 was reported on 26 Apr. The World Health Organization (WHO) declared a Phase 4 pandemic alert (sustained human-to-human transmission) in April and a Phase 5 alert (widespread human infection) shortly afterward. On 11 Jun 2009, WHO declared a Phase 6 alert, which is declared when community level outbreaks occur in geographically diverse countries, making it officially a pandemic. It was the first time the organization had called a flu pandemic in 41 years. It is unclear whether the actions of the WHO prevented new infections with the virus.

Bird Flu

"Bird flu," or avian influenza, is a contagious disease of animals caused by viruses that normally infect only birds but also, though less commonly, pigs. It is caused by H5 and H7 subtypes of the type A virus. Avian influenza viruses are very species-specific, but have crossed the species barrier to infect humans, though that has happened rarely.

There are at least 15 types of avian influenza, but the H5N1 virus, which can affect humans, has been of particular concern. Between mid-December 2003 and August 2004, poultry outbreaks caused by the H5N1 virus were reported in nine Asian countries followed by cases in Russia and Europe.

Although the risk of humans contracting bird flu is extremely low, the persistence of H5N1 in poultry populations poses two main risks for human health. The first is the risk of direct infection if the virus passes from poultry to humans. When the H5N1 virus has crossed the species barrier to infect humans, it has followed an aggressive clinical course and caused severe illness and death. Most cases have occurred in previously healthy children and young adults. A second, ever greater, risk is that the virus will change into a highly infectious form and spread easily from person to person.

Infection in humans is caused by direct contact with infected poultry or with surfaces and objects contaminated by their feces. The dangerous H5N1 virus has not hit Canada, although the H5 subtype has been found here. A low pathogenicity H5N2 avian influenza was discoverd in a commercial poultry operation in BC on 24 January 2009.

Seasonal Influenza

The active influenza strains that cause seasonal flu outbreaks change annually. Seasonal influenza may be caused by one of the influenza A viruses. Seasonal influenza tends to infect people in the extremes of age, both old and young, and people who may have compromised immune systems or other chronic health conditions. The novel type A H1N1 influenza affected people of all ages. The Spanish flu of 1918-1919 also preferentially infected young healthy people. With a new virus, people have not had time to develop immunity to it, and therefore the illness may become more widespread.

The World Health Organization monitors influenza activity and selects three strains each year that will form the base for a global flu vaccination. The flu vaccine is produced by using an organism with specific proteins called antigens, which cause the immune system to respond to and eliminate the pathogen. This provides immunity to the person receiving the vaccine. Each year there may be active influenza strains that are not included in the vaccine. For that reason, it is possible, although unlikely, for a person to develop influenza despite immunization against the illness.

It is the combined responsibility of Health Canada and the Public Health Agency of Canada to develop plans to prevent and control influenza each year. These agencies monitor influenza activity across the country, as well as make preparations for the possibility of a global pandemic of influenza in the future.

Flu vaccination is recommended for the typical influenza risk groups: the very young, the elderly, health workers and those with compromised immune systems. Vaccination programs typically begin in the autumn, just prior to the most active influenza season. It is possible, but rare, to develop an allergic reaction to the flu vaccine. A rare side effect of flu vaccination is the development of Guillain-Barré syndrome, which is an autoimmune disease that affects the nervous system, causing weakness and sensory disturbances.