Influenza occurs every winter, usually between November and February, although outbreaks can arise at any time. It is only now, with the ability to treat and manage influenza, that planning for an outbreak is feasible. The influenza virus is able to undergo genetic drift as well as shift. Genetic drift occurs when the virus genome does not create an exact copy of itself and encodes a different protein structure that is not recognised by host immune systems. Genetic shift occurs when two different strains of flu infect the same cell and major genetic reassortment ensues.
Genetic shift is believed to be the mechanism by which major pandemics and epidemics arise.
Classification of influenza is based on its surface proteins together with reference to the year of identification, place of identification and a reference number assigned by the World Health Organisation. Worldwide monitoring is not complete however, because there are few monitoring laboratories in developing countries. Identification of circulating strains of flu between the northern and southern hemispheres can predict likely infective strains for each hemisphere's winter. Vaccine manufacture is determined by the WHO to prepare the appropriate hemisphere for the virus that is most likely to be circulating. Birds seem to be the natural hosts for the flu virus and infection spreads rapidly between individuals.