Influenza (Flu)
Influenza is caused by infection with one of the influenza viruses, of which there are three, called influenza A B and C. It is mainly influenza A and B that cause major symptoms in humans. These are small viruses, contained within an envelope which has proteins on its surface that enables it to stick to cells in the human upper respiratory tract, and thereby cause infection. The incubation period, which is the time between infection and replication and the appearance of symptoms, is short at around about two days - and the severity of symptoms depends upon the immune status of the patient and the virulence of the infection. The Influenza A virus also affects seabirds, which act as a natural reservoir, and from which they can spread to affect pigs and domestic poultry as well as humans. During the course of replication in either birds or mammals the viruses change, or mutate, which is the reason why immunity does not carry on from one episode of infection to the next. This is a process called antigenic drift (the agent which causes the production of antibodies is called an antigen, and antigenic drift means they change slightly), which happens every year. More rarely there is the introduction of a new species of influenza virus from either a bird or a mammal into the human, causing large numbers of severe illness and death - a process called a pandemic which results from antigenic shift (which means the antigens have changed significantly, and none of us have very much immunity).
Influenza is a killer on its own, particularly in those who are vulnerable as a result of pre-existing disease such as heart disease or lung disease, and also because it predisposes to the development of bacterial infection such as bronchopneumonia in the elderly. It may be prevented to a certain extent by vaccination, which is offered to those at risk of complications, and there are agents available which interfere with the ability of the virus to stick on to cells and thereby reduce the chance of developing infection in the person who has been exposed. Once an infection has been established the treatment is aimed at the relief of symptoms by reducing the fever, the congestion and the pain of the aching muscles until the immune system asserts itself.
|
