- guardian.co.uk, Monday January 10 2000 13.04 GMT
Influenza is defined by the Gray Laboratory's medical dictionary as "an acute viral infection involving the respiratory tract. It is marked by inflammation of the nasal mucosa, the pharynx and conjunctiva and by headache and severe, often generalised myalgia. Fever, chills and prostration are common." The incubation period is one to three days and the disease ordinarily lasts for three to ten days.
What are the symptoms?
Self-diagnosis of flu is difficult as its classic symptoms, a fever, a sore throat, a headache and aching joints, are often confused with a common cold. You can test whether you have the flu on the NHS Direct website.
Flu differs from a common cold in the way that its viral strains rapidly appear and affect large numbers of people. Flu usually involves these initial symptoms, along with a worsening of the sore throat and muscle pains.
How does it spread?
Flu viruses enter the body through the eyes, nose or mouth. Flu is usually spread from person to person via airborne droplets or respiratory fluids, especially coughing and sneezing.
Is it fatal?
Yes. Every year flu kills thousands, particularly the elderly. Healthy people tend not to die from flu, although they can if complications develop, such as pneumonia or blood poisoning.
What I do?
If you are suffering from flu symptoms, you should stay at home, keep warm (but not over-warm), take basic painkillers such as Paracetamol to relieve your temperature, and drink plenty of water or other fluid to prevent becoming dehydrated.
Antibiotics are not effective against the flu. If your illness lasts longer than the usual three to ten days, you should see your doctor. You should also get medical advice if you are at risk of developing complications. These tend to emerge if you are old, infirm, suffer from a chronic illness or respiratory disease, or have a reduced immunity, from HIV or undergoing chemotherapy for instance.
If you haven't suffered from the virus this year and are at risk from developing severe flu, you can get a vaccination, which provides providing immunity to flu infection. 9m people in Britain have been vaccinated this winter. You need the injection every year: the vaccine is updated annually because of flu's constant mutation into different viral forms.
What is the cure?
Flu can be prevented by vaccination, but there is no cure. Flu is categorised into types A, B and C. Every year or so there are minor "drifts" in the viral strains of flu. These changes are enough to make those who suffered from - and became immune to - the previous strain susceptible to the new one.
This occurs because each flu strain has different surface antigens - which are molecules of two proteins. A person's immune system produce antibodies to fight the invading virus. These are developed specifically to attack each antigen, so when a new flu strain emerges it can circumvent the existing armoury in the body's immune system.
Large changes in the makeup of flu type A occur every decade or so. These are called "antigenic shifts" and tend to result in large epidemics.
Is this bout of flu an epidemic?
An epidemic is defined by the online medical dictionary as "occurring suddenly in numbers clearly in excess of normal expectancy, said especially of infectious diseases but applied also to any disease, injury or other health related event occurring in such outbreaks."
The government says flu reaches epidemic proportions when 400 people per 100,000 are suffering from the virus. Last week, statistics gathered from doctors across the country showed that 144 people per 100,000 had the flu.
The government's chief medical officer professor Liam Donaldson has called the current outbreak the first "serious epidemic" for 10 years. He says the figures are an underestimate because thousands are bypassing their doctors and going to the pharmacist or using the government's new NHS Direct service.
The medical profession disagree. The British Medical Association chairman Dr Ian Bogle has denied the existence of an epidemic. Dr Douglas Fleming, the director of the Birmingham research unit which calculates the official flu figures, says that flu rates are actually "very similar to the figures from last year which were not epidemic".
This year's outbreak falls well short of previous epidemics. In the last official epidemic during the winter of 1989-90, 600 people per 1,000 fell ill and 26,000 people died. In 1918 there was a flu pandemic, an epidemic that affects a wide geographic area, which killed more than 20 million - more than died in the war that had just ended.
Called "Spanish" flu, it was said to come from China, although professor John Oxford, writing in today's Daily Mail, says new research suggests it originated in a British military base in northern France.
The current virus appears to be a minor mutation, rather than a major new shift. It is called "Sydney" flu, a mutated form of type A flu which emerged in Sydney two years ago. It has been suggested that Australian rugby supporters helped spread the virus in Britain this summer.
Why has the NHS been hit so hard?
There are 1,500 intensive care beds in NHS hospitals across the country. Yesterday, there were between 20 and 31 free.
The current outbreak of flu has hit the NHS because the population is ageing. A greater proportion of the population is now elderly people, who often require hospital treatment for severe flu, than during the last major epidemic in 1989/90.
The NHS confederation says that efficiency reforms over the last decade have reduced the number of spare beds in hospitals. Many regularly run their emergency wards 95% full. This compares with 75% or less 20 years ago, which gave hospitals greater capacity to deal with sudden increases in those requiring hospital beds.
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