The discovery date of the flu virus is complicated because the disease was recognized centuries before its invisible, non-bacterial cause was identified. Influenza is an acute respiratory infection that has plagued human populations for millennia, causing yearly epidemics and devastating pandemics. The definitive discovery refers to the moment scientists successfully isolated and proved the existence of the specific viral agent responsible for the illness.
Early Recognition and Naming
Long before modern microbiology could identify the pathogen, historical records detail recurrent, widespread outbreaks of a severe respiratory illness. The name “influenza” reflects the scientific limitations of the medieval era when the cause was a profound mystery. The word entered English from the Italian influenza, meaning “influence.”
The term derived from the belief that these epidemics were caused by an intangible, atmospheric, or astrological “influence” of the stars. This concept of celestial causation was common during the 16th and 18th centuries. Documented outbreaks consistent with influenza date back as far as 1580, establishing a long history of this illness. These early accounts provided clear clinical descriptions but offered no insight into the biological origin.
The Bacterial Misdirection
The search for the causative agent intensified following the 1889–1890 pandemic, leading to a detour into bacteriology. In 1892, Richard Pfeiffer isolated a bacterium from the sputum of influenza patients, naming it Bacillus influenzae. This organism frequently appeared in the lungs of people who had died from respiratory infections following the flu.
Because this bacterium was commonly associated with severe cases, Pfeiffer and others mistakenly concluded it was the primary cause of influenza. This misidentification was solidified during the 1918–1919 pandemic, where the organism was frequently recovered from victims’ lung tissue. The misconception was so strong that the bacterium kept the name, even after it was reclassified as Haemophilus influenzae to denote its need for blood factors.
It was later understood that Haemophilus influenzae was not the cause of the flu, but a common secondary invader. The majority of deaths during the 1918 pandemic resulted from subsequent bacterial pneumonias caused by organisms like Pfeiffer’s bacillus. The true pathogen was a sub-microscopic agent that could pass through filters designed to trap bacteria, hinting at a non-bacterial cause.
Isolating the Viral Agent
The true nature of influenza as a viral disease was confirmed in 1933, marking the definitive discovery date for the pathogen. This breakthrough occurred at the National Institute for Medical Research in London, led by Patrick P. Laidlaw, Christopher H. Andrewes, and Wilson Smith. They hypothesized the cause was a “filter-passing virus,” smaller than any known bacterium.
The team collected filtered throat washings from people suffering from an influenza outbreak. To isolate the unknown agent, they needed a suitable animal model. They succeeded by instilling the filtered material into the noses of ferrets.
Within days, the ferrets exhibited classic signs of influenza, proving the material contained a transmissible agent. The researchers demonstrated that this agent was filterable and could be passed serially from one ferret to another.
The isolation of this agent, classified as the first human Influenza A virus, conclusively ended the misdirection toward bacteria. The ferrets proved to be an excellent model, confirming the virus’s ability to transmit to humans. This successful isolation in 1933 provided the first tool for studying the virus and creating protective vaccines.
Post-Discovery Classification
The initial isolation of Influenza A in 1933 led directly to the establishment of a formal classification system. The next major type, Influenza B virus, was identified in 1940 during an outbreak in North America. Researchers determined that the new virus lacked antigenic cross-reactivity with Influenza A, necessitating a separate designation.
The discovery of Influenza C virus came in 1947, expanding the understanding of the Orthomyxoviridae family. Classification into Types A, B, and C was necessary because each type possesses distinct genetic and antigenic properties. Type A viruses cause all flu pandemics, while Type B viruses cause seasonal epidemics. This system allows scientists to track circulating strains and accurately formulate seasonal vaccines.