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Influenza - commonly called the "flu" - is one of the oldest and most common diseases known to man. It can also be one of the deadliest. Influenza was first described by Hippocrates in 412 BC and the first well-described pandemic of influenza-like disease occurred in 1580. Since that time, 31 such possible influenza pandemics have been documented, with three occurring in this century: in 1918, 1957 and 1968. The disease today still affects large sections of the population each year. Its ability to kill stems from the fact that the virus can mutate quickly, often producing new strains against which human beings have no immunity. When this occurs, mortality from influenza can be staggering such as, for example, during the "Spanish Flu" pandemic of 1918-1920, when an at least 20 million people died from influenza.

The Disease: Influenza is an acute respiratory illness caused by influenza viruses A and B. On clinical grounds alone it cannot be distinguished from other acute respiratory infection; laboratory tests are necessary. It is most often a mild viral infection transmitted by respiratory secretions through sneezing or coughing.

Symptoms: fever (often higher in children), cough, sore throat, runny or stuffy nose, headache, muscle aches, and often extreme fatigue. Most people recover completely within 1-2 weeks. However, compared with other viral respiratory infections like common colds, influenza causes more severe complications such as pneumonia, particularly in children, elderly people and other vulnerable groups.

Epidemiology: Influenza is a disease that comes in seasonal epidemics and occurs due to minor changes in the influenza viral antigenic proteins (antigenic drift). These account for most of the changes in the viruses from one season to another.

Occurrence of epidemics has been associated with influenza A viruses. Influenza B viruses also cause epidemics and are largely restricted to human beings, whereas all human influenza A viruses infect avian species. Only a few antigenic subtypes of influenza A infect man and other animals, in particular pigs and horses. Currently there are three different influenza strains circulating worldwide; two subtypes of influenza A and one of influenza B.

Influenza also occurs as worldwide epidemics - called pandemics - due to major antigenic changes (antigenic shift) that are independent of season. Such antigenic shifts occurs only occasionally. When it happens, however, pandemic influenza, which could affect large portions of the population, results. This is because of the absence of immunity against the "new" virus. Before World War 1 influenza was not considered a major public health threat. But that changed with the great "Spanish flu" pandemic in 1918-1920, which was caused by influenza A(H1N1).

The pandemics in 1957 ("Asian flu"), caused by influenza A(H2N2), and 1968 ("Hong Kong flu"), caused by influenza A(H2N2), together killed more than 1.5 million people and caused an estimated $32 billion in economic damages worldwide due productivity losses and medical expenses.

There is evidence that the viruses which caused these pandemics originated from animals (1918 = swine, 1957 and 1968 = avian strains). In 1976, a new influenza virus from pigs caused human infections and severe illness, while in 1997-1998, an outbreak of influenza whose origins were avian occurred in Hong Kong, Special Administrative Region of China. A vaccine against swine influenza was developed and administered in some countries in 1976, although no pandemic in fact occurred. An experimental vaccine is being developed in response to the recent avian flu outbreak.

Diagnosis: Influenza infections can be diagnosed by serology using haemagglutinin-inhibition tests to detect antibodies during acute infections. More immediate diagnosis may be achieved by the direct detection of viral antigens in nasal secretions by inmmunoflourescence test. Diagnosis could also be done by polymerase chain reaction (PCR) or by antigen capture ELISA with monoclonal antibody to the nucleoprotein. Culture of the influenza viruses from nasopharyngeal secretions are preferred for further identification of strains.

Prevention and Control: Viruses causing influenza continually change over time, which enables them to evade the immune system of its host. This influenza viral property plus the existence of an aquatic bird reservoir of all known influenza A subtypes make human beings susceptible to influenza virus infection. Rapid changes in the most commonly circulating types of influenza virus necessitate annual changes in the composition of the vaccine. Prevention and control are thus the only realistic means of dealing with influenza.