Influenza, commonly called "the flu", is a contagious disease caused by viruses that infect the respiratory tract including nose, throat, and lungs. Influenza causes severe illness and life threatening complications in many people.

Influenza is caused by three types of viruses called influenza A, B, and C. Influenza types A and B are responsible for the respiratory disease that occurs almost every winter. Influenza type C usually causes a very mild disease often without symptoms.

The symptoms of influenza include fever, cough, sore throat, runny nose, headache, muscle aches, and fatigue.

Most people who get the flu recover completely. However, some people especially the elderly and those with chronic health problems can develop serious complications. These include pneumonia and aggravation of pre-existing medical conditions such as congestive heart failure, asthma, or diabetes.

The influenza viruses mainly are spread from person to person through droplets produced while coughing or sneezing. Droplets of an infected person are propelled by coughing and sneezing into the air and are deposited on the mouth or nose of people nearby. This droplet transmission of the flu is a kind of contact transmission.

The influenza viruses can also be transmitted by indirect contact by touching a contaminated object or surface and then touching your own mouth, eyes or nose before washing your hands. This is also called fomite transmission - a fomite is any surface or inanimate thing (door knobs, telephones, towels, money, clothing, dishes, books, etc.) that has a contagious or infective agent after an infected person contaminated it by touching it or sneezing on it. Viruses can survive on surfaces - longer on hard, impermeable surfaces than on porous surfaces. The viruses can still be infective for two hours and maybe up to eight hours. It is easier to catch the common cold than influenza by fomite transmission because some of the "cold" viruses (rhinoviruses) have much smaller infectious doses than the “flu” viruses.

Influenza can be prevented by annual vaccination. In Canada, the National Advisory Committee on Immunization (NACI) recommends the influenza vaccine for the following people:

• Adults and children with chronic heart or lung disease.
• People of any age who are residents of nursing homes and other chronic care facilities.
• People over the age of 65 years.
• People with chronic conditions such as diabetes, cancer, and kidney disease.

The vaccine is also recommended for people who are capable of transmitting influenza to those at high risk. For example, health care workers and those providing essential community services.

People wishing to protect themselves against influenza should also consider vaccination even if they are not in a high risk group.

To prevent the transmission of influenza use the following hygiene practices:

• Cover your mouth and nose when coughing or sneezing.
• Use tissues to contain respiratory droplets.
• Wash hands after contact with respiratory droplets and contaminated objects. (Frequent hand washing with appropriate handcare to prevent skin irritation is always a good idea.)

Good personal hygiene practices will reduce the risk of infection. However, the only effective method of prevention is vaccination.