system
Given the following post, respond as if you're commenting on Reddit:
human
How did scientists during the 1918-1919 Influenza Pandemic know what a virus was?

Medical science had come a long way throughout the latter half of the 19th century, let alone up to WWI. The germ theory was generally accepted, and scientists were mostly on the same page about how disease outbreaks started with microorganisms. They could even observe and describe the shape of certain bacteria with microscopes. But how did they know what viruses were? They’re too small to physically observe with early 20th century technology, and you can’t culture them in the same manner as bacteria. A lot of work was done during the Influenza Pandemic around finding the causative agent. However, much of this work mistakenly focused on bacteria that colonize the lungs as a consequence of the initial influenza infection. They clearly knew that viruses existed (IIRC scientists had attributed polio to a viral, rather than bacteria, infection at that point) but how did they know this given the limits of scientific methods at the time?
gpt
In the late 19th century they produced a filter, called the Chamberland filter, able to filter out bacteria.  This led to the discovery of both viruses and bacterial toxins in the late 19th century.  Scientists had determined that viruses (like tobacco mosaic virus) could remain infectious even when filtered through a filter that removed all bacteria.  So they knew there was something smaller than bacteria that was capable of causing disease.  "Virus" was the name given, but they still didn't know much about these new very-small particles.

Influenza was believed to be a virus, but it was not discovered to be a virus until the 1930s.

But scientists in 1917 were **far from certain** about the cause of "The Spanish flu."  Many at the time believed the causative agent was the gram-negative bacterium now called H. influenzae but back then known as "Pfeiffer's bacillus."  Others, however, found that secretions from people with Spanish influenza were still infectious after passing through a Chamberland filter.  But other scientists had difficulty reproducing their results.

Influenza was diagnosed clinically, without the term indicating a particular infectious agent.  They didn't know how many agents might be involved.

It wasn't until experiments 10 years later that flu would be *proven* to be a virus.  A doctor, Shope, was looking into epidemic outbreaks among pigs that appeared identical to influenza.  They took samples from infected pigs and isolated Pfeiffer's bacterium - but the bacterium failed to transmit the infection.  But when they filtered the samples, the filtrate *did cause infection*.  It was thus a virus.  He also demonstrated that sera from humans infected with the 1918 pandemic flu could neutralize the pig virus - and thus, the Spanish flu agent was likely the same virus.

This research allowed for the isolation of influenza a few years later.