Abstract

In April 2009, the Mexican Secretariat of Health reported an outbreak of respiratory disease. In affected patients, a novel swineorigin influenza A(H1N1) virus (S-OIV) with molecular features of North American and Eurasian swine, avian, and human influenza viruses was found. On June 11, 2009, The World Health Organization declared that an influenza pandemic was under way.

Influenza is an acute respiratory illness caused by an influenza virus, usually of type A or B. Influenza may be clinically indistinguishable from diseases caused by other respiratory viruses. Influenza derives its importance from the rapidity with which epidemics evolve. Infl uenza type A viruses cause annual epidemics, and occasional  pandemics that have claimed the lives of millions. The emergence of inf luenza A (H1N1) 92 years ago in 1918 led to a disastrous pandemic.

The virus is efficiently transmitted between individuals through droplets of aerosols generated by coughs and sneezes. Aerosol spread of infl uenza virus is dependent upon both ambient relative humidity and temperature, both cold and dry conditions favouring transmission. S-OIV infection can cause severe illness, the acute respiratory distress syndrome, and death in previously healthy persons who are young to middle-aged, which is untypical for seasonal influenza infection. Our review focuses on the epidemiology of influenza and the human immunity response to the virus.