In the early 1930s, Richard Shope isolated influenza virus from infected pigs. Shope's finding was quickly followed by the isolation of the influenza virus from humans, proving that a virus—not a bacterium, as was widely believed—caused influenza.
The filtration question
When history's deadliest influenza pandemic began in 1918, most scientists believed that Pfeiffer's bacillus caused influenza. With the lethality of this outbreak (which killed an estimated 20 to 100 million worldwide) came urgency—researchers around the world began to search for Pfeiffer's bacillus in patients, hoping to develop antisera and vaccines that would protect against infection. In many patients, but not all, the bacteria were found. Failures to isolate B. influenzae (now known as Haemophilus influenzae) were largely chalked up to inadequate technique, as the bacteria were notoriously difficult to culture (2).
The first potential blow to Pfeiffer's theory came from Peter Olitsky and Frederick Gates at The Rockefeller Institute. Olitsky and Gates took nasal secretions from patients infected with the 1918 flu and passed them through Berkefeld filters, which exclude bacteria. The infectious agent—which caused lung disease in rabbits—passed through the filter, suggesting that it was not a bacterium (3, 4). Although the duo had perhaps isolated the influenza virus (which they nevertheless referred to as an atypical bacterium called Bacterium pneumosintes), other researchers could not reproduce their results. One of the doubters was Oswald Avery (Rockefeller Institute), who developed a culture media—chocolate agar—that optimized the growing conditions for B. influenzae and thus minimized false negative results from patient samples. Thus, the idea that flu was transmitted by a filterable agent (or virus) was dismissed.
Insights from pigs
Olitsky and Gates would not be vindicated until a decade later, when Shope—a young physician from Iowa then working on hog cholera at the Rockefeller Institute—turned his attention to swine influenza.
Pig farmers in Iowa had reported two outbreaks—one in 1918 and another in 1929—of a highly contagious, influenza-like disease among their animals. The disease bore such a remarkable resemblance to human flu that it was named swine influenza. Shope and his mentor Paul Lewis took mucus and lung samples from the infected pigs and attempted to isolate the disease-causing agent. They quickly isolated a bacterium that looked exactly like Pfeiffer's human bacterium (and was thus called B. influenzae suis), but when they injected the bacteria into pigs, it caused no disease (5).
Shope then filtered the samples and, like Olitsky and Gates, found that the filtrate contained the infectious agent. Shope's filtrate caused a highly contagious, influenza-like disease in pigs—albeit a more mild one than seen in naturally-infected pigs. Mixing the filtrate with the bacterium reproduced the severe disease. He concluded—correctly—that the filterable agent caused the infection, which then facilitated secondary infection with the bacterium (6). Shope published his results in a series of papers in The Journal of Experimental Medicine (5, 6).
Using Shope's technique, Wilson Smith, Christopher Andrewes, and Patrick Laidlaw (National Institute for Medical Research, UK) soon isolated the virus from humans (7), laying to rest any lingering doubts about the nature of the flu-inducing agent.
Both Shope and the British trio later demonstrated that sera from humans that were infected with the 1918 flu virus could neutralize the pig virus, leading them to conclude that the swine virus was a surviving form of the 1918 human pandemic virus (8, 9). In fact, a related strain of flu still circulates among pigs today.