Influenza is a highly contagious respiratory disease caused by influenza viruses. The name originated in 15th century Italy, from an epidemic of the disease attributed to “influence of the stars.” The first pandemic (epidemic occurring worldwide, crossing international boundaries) that was clearly caused by influenza virus happened in 1580.

At least four pandemics of influenza occurred throughout 19th century, while three of them occurred in the 20th century. Each one differed from the others with respect to viral subtypes, epidemiology and disease severity. The first pandemic of the 21st century occurred between 2009 and 2010.

Three pandemics of the 20th century

Between May of 1918 and April of 1919, a highly virulent H1N1 strain of the influenza virus, also known as the Spanish flu, killed at least 50 million people worldwide – five times more when compared to those who died in the World War One. A majority of deaths was a result of secondary bacterial infections (such as pneumonia) that were particularly serious in those suffering from underlying pulmonary illnesses, such as tuberculosis.

Unlike earlier pandemics and seasonal influenza outbreaks, the 1918 pandemic resulted in high mortality rates among healthy adults. In fact, the illness and mortality rates were highest among adults between 20 and 40 years of age. Epidemiologic evidence strongly suggested that the causative virus was moving from humans to swine.

The 1957-1958 pandemic has often been dubbed as the first pandemic in the era of modern virology, as it was the first time the rapid global spread of an influenza virus was available for laboratory evaluation. Also known as the Asian flu, this pandemic resulted in two million deaths worldwide.

Although the percentage of infected people was high, the resulting disease was relatively mild compared to the Spanish flu, thus resulting in fewer deaths. Studies revealed that the strain responsible for the Asian flu (H2N2) arose by genetic reassortment of a bird virus.

The Hong Kong 1968-1969 influenza pandemic almost passed undetected, even though it spread throughout the world in less than two years, given the already significant increase in intercontinental travel. The cause was a H3N2 strain of the influenza A virus and the death toll was significantly lower when compared to Spanish and Asian flu, making it the mildest pandemic in the 20th century.

2009 influenza pandemic

The first influenza pandemic of the 21st century, initially reported in April 2009 in Mexico, swept through the Northern Hemisphere in two waves. This pandemic raised global concerns rooted in experience from previous pandemics – especially from the notorious Spanish flu which killed 50 millions of people.

Virus responsible for the pandemic was a novel H1N1 strain derived from human, swine and avian strains, hence it was informally termed “swine flu”. Cases of influenza caused by this strain have been confirmed in most of the countries throughout the world by the World Health Organization (WHO).

It caused widespread and quite unusual outbreaks of the disease in the summer and very high levels of disease during the winter. It was also characterized by a noteworthy dominance of the pandemic virus over other seasonal influenza viruses, and also by an unusual clinical pattern where the most severe cases occurred in younger age groups.

Between 150 and 575 thousand people died from 2009 H1N1 virus infection during the first year of the virus circulation. A disproportionate number of deaths occurred in Africa and Southeast Asia, where access to prevention and treatment resources were limited.

Although this influenza pandemic was hard on the people younger than 65 years of age, the impact on the global population overall during the first year was less severe than in previous pandemics. The impact of illness was also minimized by adequate prevention, as 80 million people were vaccinated against H1N1.

On August 10 2010, WHO announced that the H1N1 influenza incident has moved into the post-pandemic period. Based on the data about past pandemics, the 2009 virus is expected to linger as a seasonal virus for years to come. And while the level of concern is not high anymore, alertness on the part of national health authorities remains a pivotal issue.