by Alison Bowler

Created on: May 01, 2009   Last Updated: August 31, 2009

Many illnesses, such as colds, circulate within the human population through out the year. Influenza or flu, however, is a seasonal illness. The season in which it is active is winter. Cases of seasonal influenza are caused by strains of either the Influenza A virus or Influenza B virus. Pandemic strains of Influenza A, such as the 2009 swine influenza and the 1918 Spanish flu, do not always show this seasonal distribution of infection and cases of influenza occur throughout the summer months in pandemic years.

In the Northern Hemisphere, the first cases of influenza occur in late October or early November. The incidence of flu remains at quite a low level until December. The number of cases of flu rises through December peaking between January and March. During April, case numbers fall and the last few cases occur in May.

In the Southern Hemisphere, influenza is also a winter illness usually striking between May and October.

Influenza infection shows no predilection for either sex and can infect any age group.

The symptoms of influenza are: fever(in adults this can be between 100 F to 104 F (38 C to 40C) children will often have an even higher temperature), respiratory tract irritation (shown as a harsh cough, sore throat and occasionally a runny or stuffy nose), headache, myalgia and arthralgia. Patients will experience fatigue, which can sometimes be extreme in nature. Some patients, particularly children, may experience gastrointestinal symptoms such as nausea, diarrhea and vomiting. Patients are usually symptomatic for five to ten days.

Occasionally patients may also be affected by one of a number of potentially life threatening complications of influenza. These complications may be caused by the virus or by opportunistic bacteria and include pneumonia or bronchitis, the brain infection encephalitis, sinusitis and ear infections.

Some groups of people are more prone to the complications of influenza infections. These groups are the elderly, people with chronic conditions such as diabetes, asthmatics, pregnant women, children under the age of two, immunocompromised patients and patients with chronic heart, lung or kidney diseases.

The seasonal nature of influenza infections allows for the production and distribution of a vaccine to prevent the infection. In non-pandemic years, the vaccine normally offers about a seventy percent protection rate against influenza infection. Owing to the changeable genetic nature of influenza viruses, a new vaccine formula must be prepared and administered every year to be effective.

In the Northern Hemisphere, the flu vaccination program normally starts in September and runs through to November. This program targets certain groups. In addition to the high-risk groups, medical personnel, carers and close family members of high-risk patients are routinely offered the vaccination. Other groups include residents and staff of long-term care institutions and essential service workers. Children or teenagers on long-term aspirin treatment may develop the fatal condition Reyes syndrome if they catch influenza so they are also given the vaccination.

There are other viral infections, which can give flu-like symptoms. The parainfluenza viruses can infect people throughout the year and account for some of people who report developing "summer flu".

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