Influenza


Influenza or as it is commonly known the flu is a contagious disease caused
by an RNA virus of the orthomyxoviridae family. The name comes from the old
medical belief in unfavourable astrological influences as the cause of the
disease.

There are three types of the virus:

   * Influenza A viruses that infect mammals (humans, pigs, ferrets, horses)
     and birds
   * Influenza B viruses that infect only humans
   * Influenza C viruses that infect only humans

The A type of virus is the most virulent and causes epidemics and pandemics.
Where a finer grained classification of the virus strain is needed, this is
done by reference to the structural forms of two viral proteins
(haemaglutinin and neuraminidase) that are essential to the virus' life
cycle. Thus one might speak of H1N1 or H3N2 viral strains.

The virus attacks the respiratory tract, is transmitted from person to
person by droplets, and causes the following symptoms:

   * Fever
   * Headache
   * Tiredness (can be extreme)
   * Dry cough
   * Sore throat
   * Nasal congestion and sneezing
   * Body aches

Although a lot of people in the western world will often call a heavy cold
"flu", you know when you have got real influenza as its effects are much
more severe and last longer. Typically influenza takes about 1-2 weeks to
recover from. Flu can be a killer disease, especially for the weak, old or
chronically ill. Some flu pandemics have killed millions of people, for
example the "Spanish Flu" pandemic of 1918-1919, which is believed to have
killed more people in total than World War I.

Influenza is an extremely variable disease: similar viruses are found in
pigs and domestic fowl. In areas where there are high concentrations of
humans, pigs and birds in close proximity, such as parts of Asia,
simultaneous infections across species enable genetic material to be
exchanged between the various strains of flu. This appears to be the
principal method by which new infectious strains arise. It is believed that
sooner or later, a recombination may occur to produce a strain as lethal as
the 1918 virus. In late 1997, a new strain of influenza originating from
chickens infected 18 people in Hong Kong, of whom 6 died. This strain did
not appear to be readily transmissible from human to human, but such a high
mortality rate, and the possibility of a further recombination to make it
more infectious, meant that the risk was considered so great that all
domestic poultry in Hong Kong was slaughtered. As of April 2002, there have
been no further human infections with this particular strain.

Prevention

It is possible to get vaccinated against Influenza, however due to the high
mutability of the virus the vaccine is usually only good for a year or so.
The World Health Organization co-ordinates the contents of the vaccine each
year to contain the most likely strains of the virus to attack the next
year. The vaccine is usually recommended for anyone in a high-risk group who
may suffer complications from a real flu attack.