Influenza – A history of epidemics

Influenza is a disease that is caused by a number of strains of related viruses and is particularly prevalent in countries that have a severe winter – in large parts of Europe and North America, for example. There are records of influenza epidemics in the past that have caused widespread infections and deaths. The severest recorded in recent history is the ‘Spanish flu’ epidemic that raged just after the First World War. Between 1918 and 1920, the Spanish flu pandemic (ie, an epidemic that established itself in several regions of the world) infected 500 million people across the world and killed an estimated 50 to 100 million people. Since then, though we have not encountered a pandemic that equals the severity of the Spanish flu, there have been periodic epidemics and pandemics affecting different parts of the world. The most reason pandemic was in 2009, caused by the swine flu (H1N1) virus. The World Health Organisation (WHO) issued a level 6 pandemic warning in 2009 in the midst of widespread fears that it would rival the Spanish flu pandemic in terms of effects on the global population. However the 2009 pandemic proved to be much milder than anticipated. Retrospective studies now indicate that almost a quarter of the world’s population was infected by the H1N1 virus between 2009 and 2010, indicating that in terms of percentage of the world’s population infected, the 2009 pandemic was quite similar to the 1918 pandemic. Fortunately the disease was of much lower severity, and studies show that the infection led to about 2,00,000 deaths globally.

What the history of influenza epidemics and pandemics show is that there is a huge spike in the number of infections when a new variant of the influenza virus surfaces. This was the case in both the 1918 and the 2009 pandemics. As we have indicated earlier, the new variant of the virus in 2009 probably arose because of the conditions of industrial style animal husbandry, leading to the development of a new variant that incorporated genetic material from viruses that cause disease in pigs. It must be understood, however, that there is a spike (though not as pronounced as for a new variant) in the cases of ‘seasonal’ influenza in particular seasons. Seasonal flu is caused by a number of existing variants of the influenza virus that are usually present in the environment. In colder climates the spike in influenza incidence usually takes place in autumn and winter, while in tropical countries such as India, the spikes occur in spring and autumn.

The seasonal increases in incidence of infection are related to specific climatic conditions that the influenza virus finds particularly conducive and to conditions of living that allow quick transmission of the virus.