An influenza pandemic is an epidemic of an influenza virus that spreads on a worldwide scale and infects a large proportion of the human population. An influenza pandemic can occur when a non-human (novel) influenza virus gains the ability for efficient and sustained human-to-human transmission and then spreads globally. Influenza viruses that have the potential to cause a pandemic are referred to as ‘influenza viruses with pandemic potential.
Throughout the 20th century, several influenza pandemics occurred. The 1918–20 Spanish influenza pandemic was by far the most devastating on a global scale, and being responsible for the deaths of estimated approximately 50-100 million people. The pandemics after 1918 (such as 1957–58 and 1968–69) were milder and less severe. The 2009 influenza A H1N1 pandemic was the first event that fulfilled the criteria of Public Health Emergency of International Concern (PHEIC) under IHR (2005), and it was also much less severe than 1918 pandemic. In the meantime, we all have to be prepared for the possibility of the coming influenza pandemic that might occur in the future.