Focus on: Influenza  As the Department of Health launches this
year's flu vaccination campaign, Asda supermarket has announced
that it, too, will be offering the jab to 35,000 customers at a
cut-price rate.  But although flu affects more than half a
million people so badly that they see a doctor, the vaccine is
not suitable for everyone.  So who is most at risk and is there
any alternative treatment? Find out with our guide below.  What
causes it?  Flu is caused by the influenza virus, which spreads
in tiny water droplets when people cough. There are three
strains, A, B and C, yet influenza types A and B are mainly
responsible for the epidemics of illness that strike each
winter.  'When inhaled, the viruses enter cells lining the
airways of the lungs,' explains Dr Douglas Flemming, who runs a
flu monitoring unit at the Northfield Health Centre, Birmingham.
'They then multiply and spread, infecting the lung surface,
causing inflammation and a build up of mucous. The immune system
responds by sending antibodie