What is influenza?
Influenza is a highly infectious viral disease. It is caused by various types of influenza viral strains.
In Hong Kong, the disease is more common in periods from January to March and from July to August.
The virus
Three types of influenza viruses are recognized: A, B and C. Influenza A viruses are further categorized into subtypes on the basis of two surface antigens: haemagglutinin (H) and neuraminidase (N).
Infection caused by influenza A viruses is more common. In Hong Kong, the two subtypes of influenza A, H1N1 and H3N2, are more commonly seen.
New subtype variants appear from time to time and at irregular intervals. This is responsible for seasonal outbreaks and makes periodic reformulation of the influenza vaccine necessary.
Symptoms
Influenza is an acute illness of the respiratory tract, characterized by fever, headache, muscle ache, runny nose, cough and sore throat. The incubation period for influenza is usually about 1-3 days.
Transmission
The disease spreads from person to person by respiratory droplets generated by coughing and sneezing. Occasionally a person may become infected by touching objects carrying the virus on the surface and then transferring it to their mouth or nose.
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