Influenza is a highly contagious, airborne respiratory tract
infection associated with a significant disease burden. The
annual “mild” influenza epidemics caused by antigenic drift
of the virus affects 10–20% of the world’s population with
up to 5 million cases of serious illness and 500,000 deaths
(http://www.who.int/vaccine research/diseases/ari/en). At various intervals, new influenza subtypes emerge against
which no immunity exists in the human population and these
may cause global pandemics with an even higher disease toll.
The current outbreak of a new influenza subtype A (H5N1),
which can be directly, although at this time rarely, transmitted
from birds to humans [1], is an example of a potential pandemic
flu threat, which has currently reached phase 3 of 6 in
the WHO delineation of a flu pandemic (http://www.who.int/
csr/disease/avian influenza/phase/en/index.html). It is highly
pathogenic in birds, and when transmitted to humans it
carries a mortality of over 50% [2]. So far, all the resulting
human cases have been in close contact with infected flocks
and there is no known example of a human-to-human
transmission of the current H5N1 bird flu. However, it
is the largest and most severe influenza epidemic ever
registered among birds; since 2003, it has spread rapidly
to poultry in many countries in Asia and most recently it
seems to have established itself in Turkey. The size of this
virus repertoire has caused concerns that re-assortment or
mutations of influenza genes occurring in bird populations,
or in infected humans, eventually will generate a virus that
can be transmitted from person to person causing a highly
contagious, and potentially devastating, pandemic [3].