The term “Irish Flu” is a historical, localized name for the global influenza pandemic that swept across the world at the end of the First World War. It is not a distinct medical illness but rather a regional identifier for this catastrophic event. The pandemic left a deep mark on the country’s social and political landscape.
Identifying the 1918 Global Pandemic
The disease referred to as the Irish Flu was biologically identical to the global outbreak commonly known elsewhere as the Spanish Flu. This pandemic was caused by an exceptionally virulent strain of the Influenza A virus, specifically the H1N1 subtype, which contained genes of avian origin. This particular virus was characterized by its rapid onset and severe respiratory symptoms.
Globally, the virus spread in multiple waves between 1918 and 1919, eventually infecting an estimated one-third of the world’s population. The pandemic caused at least 50 million deaths worldwide, making it the deadliest influenza event on record. A distinguishing feature of this H1N1 strain was its unusually high mortality rate among healthy young adults between the ages of 20 and 40.
This demographic, typically resilient to seasonal influenza, suffered high death rates, possibly due to an overreaction of their robust immune systems, sometimes called a cytokine storm. The lack of widely available antibiotics meant secondary bacterial infections, particularly pneumonia, were often fatal complications for the infected. The global movement of troops and civilians during and immediately following the First World War significantly accelerated the virus’s spread across continents.
The Unique Context in Ireland
The pandemic arrived in Ireland in the spring of 1918, likely carried by returning soldiers and naval movements, and then spread rapidly through the port cities. The outbreak here mirrored the global pattern, hitting hardest in a devastating second wave during the autumn of 1918. In total, the influenza infected approximately 800,000 people across the island.
The death toll reached an estimated 23,000 people, a significant loss of life that occurred during a period of intense political upheaval and social change. Wartime conditions contributed to the severity of the outbreak, as many doctors and nurses were serving abroad, leaving medical resources strained. Overcrowding in cities and poor sanitation also facilitated the swift transmission of the respiratory virus.
The name “Irish Flu,” along with other local monikers like “Black Flu” or “Flanders Grippe,” arose from this intense, shared experience. The lack of effective treatment and the swift nature of the illness made it a defining local event. The resulting disruption affected everything from public transport to political activism.