The disease is caused by a VIRUS of the influenza group. There are at least three types of influenza virus, known respectively as A, B and C. One of their most characteristic features is that infection with one type provides no protection against another. Equally important is the ease with which the influenza virus can change its genetic make-up. It is these two characteristics which explain why one attack of influenza provides little, if any, protection against a subsequent attack, and why it is so difficult to prepare an effective vaccine against the disease.

In 1997 and 2004, outbreaks of Chinese avian influenza caused alarm, as did H1N1 (‘Swine flu’) in 2009. The influenza virus had apparently jumped species – probably from chickens and pigs – to infect some people.

The incubation period of influenza A and B is 2–3 days, and the disease is characterised by sudden onset. In most cases this is followed by a short, sharp FEBRILE illness of 2–4 days’ duration, associated with headache, prostration, generalised aching, and respiratory symptoms. The latter are usually restricted to the upper respiratory tract, and consist of signs of irritation of the nose, pharynx and larynx. There may be nosebleeds, and a dry, hacking cough is often a prominent and troublesome symptom. The fever is usually REMITTENT and the temperature seldom exceeds 39.4 °C (103 °F), tending to fluctuate between 38.3 and 39.4 °C (101 and 103 °F).

The most serious complication is infection of the lungs. This infection is usually due to organisms other than the influenza virus, and is a complication which can have serious results in elderly people.

The very severe form of ’flu which tends to occur during pandemics – and which was so common during the 1918–19 pandemic – is characterised by the rapid onset of broncho-pneumonia and severe prostration.

Convalescence following influenza tends to be prolonged. Even after an attack of average severity there tends to be a period of weakness and depression.

Expert opinion is still divided as to the real value of influenza vaccine in preventing the disease. Part of the trouble is that there is little value in giving any vaccine until it is known which particular virus is causing the infection. As this varies from winter to winter, and as the protection given by vaccine does not exceed one year, it is obviously not worthwhile attempting to vaccinate the whole community. The general rule therefore is that, unless there is any evidence that a particularly virulent type of virus is responsible, only the most vulnerable should be immunised – such as children in boarding schools, elderly people, and people who suffer from chronic bronchitis or asthma, chronic heart disease, renal failure, diabetes mellitus or immunosuppression (see under separate entries). In the face of an epidemic, people in key positions, such as doctors, nurses and those concerned with public safety, transport and other public utilities, should be vaccinated.

For an uncomplicated attack of influenza, treatment is symptomatic: that is, rest in bed, ANALGESICS to relieve the pain, sedatives, and a light diet. A linctus is useful to soothe a troublesome cough. The best analgesics are ASPIRIN (adults only) or PARACETAMOL. If the lungs become infected, antibiotics might be required because such an infection is usually due to secondary invasion of the affected lung by bacteria antiviral agents, such as ostelamivir have been used in attempts to deal with a threatened pandemic but evidence for effectiveness is uncertain beyond a minor reduction in the duration and severity of symptoms.