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Influenza is spread mainly from person to person by droplet infection or droplet nuclei created by sneezing, coughing or talking. The portal of entry of the virus is the respiratory tract.

Incubation Period for Influenza
The incubation period is about 18 to 72 hours.

Pathogenesis and Clinical Features of Influenza
The virus enters the respiratory tract and causes inflammation and necrosis of the superficial epithelium of the tracheal and bronchial mucosa, followed by secondary bacterial invasion. There is no viraemia. Both viruses cause the same symptoms – fever, chills, aches and pains, coughing and generalized weakness. Fever lasts from one to five days, averaging about three days in adults. The most dreaded complication is pneumonia, which should be suspected if fever persists beyond four or five days, or recurs abruptly after convalescence.

Diagnosis of Influenza
Virus isolation
Virus isolation
Laboratory Diagnosis of Influenza
Since clinical diagnosis is difficult except during epidemics, laboratory methods are needed to confirm the diagnosis. These are:
Virus Isolation
Nasopharyngeal secretions are the best specimens for obtaining large quantities of virus–infected cells. The virus can be detected by the indirect fluorescent antibody technique. However, egg inoculation is required for virus isolation and antigenic analysis.

Paired Sera
A sero diagnosis of influenza A or B can be made by the examination of two serum specimens from a patient. One taken as early as possible in the acute phase of the disease (not later than the fifth day), and another taken about 10 to 14 days after the onset, i.e. the convalescent stage of illness. The titer of influenza antibodies in the human sera is so variable that only by detecting a rise in Complement Fixing (CF) antibodies during the course of illness, can a diagnosis be established. Hence, the need for two specimens. Fourfold or greater rise in titer are considered diagnostic of infection. At least 2ml, preferably 5ml, of each serum should be sent, the serum being removed from the clot before dispatch. The two sera should be sent together to the testing laboratory (the first serum should be kept in the refrigerator till the second is available to send with it). Serological examination is fairly satisfactory but slower, and does not provide information about the new strains which are wanted for study in relation to the spread of the disease.

In India, facilities for isolation of the influenza virus are available at:
Government of India Influenza Center, Pasteur Institute, Coonoor, South India.