I am not a historian. I am, however, a professor of anatomic and clinical pathology, and I happen to have a collection of old reference books on the topic. Of special significance is my complete collection of the Annual of the Universal Medical Sciences of 1892, which gives a window into the knowledge of diagnostic medicine and pathology around that time.

First, germ theory.

This is an exciting period for bacteriology. Robort Koch had published his work in 1877 of photomicrographs of bacteria using the latest staining techniques (some of which are still used to this day). His work in bacterial cultures, staining techniques, and photomicrography, along Ernst Abbe's work in oil immersion and microscope condensers allowed for the first true analysis of bacteria associated with disease. Koch published his work on the anthrax bacterium in 1876, and further work on tuberculosis in 1882. In 1881, he published "Methods for the Study of Pathogenic Organisms". Koch's postulates were published in 1884, which laid out the basics of germ theory.

So, by 1918, germ theory was well accepted among medical professionals. The medical community was practically giddy over the concept, and there was an explosion of research into the topic.

Second, viruses.

At this point, the concept of a "virus" had been proposed, and the actions of viruses had been documented, but the details of what a virus was hadn't been discovered. It was known that something that fulfilled Koch's postulates existed, and that it was not stopped by filters that would prevent bacteria. Medical professionals knew that there were diseases that couldn't be explained by bacteria but still followed the patterns of infectious organisms. A lay-person probably didn't know much about it, other than maybe having an idea that it was something that couldn't be found or was too small to filter out.

Third, what he could have been testing for.

Again, bacterial cultures did exist, and techniques for isolating bacteria were well-published. Blood and urine samples are still used to this day to determine the overall health of the body and can provide countless indicators of disease. Although immunology was still a young field, it was known that increased leukocytes (white blood cells) were associated with both disease and recovery. Urine and blood specimens were routinely tested for evidence of disease in almost any part of the body, including influenza.

Influenza was not an unknown disease. From the Annual of Universal Medical Sciences (1892):

"Adler concludes, that the specific of infection (a microbe as yet not definitely described) first invades the blood, since on this hypothesis alone can we account for the account appearance of the symptoms and the affectation of the organism, simultaneously, in every part..."

The text continues with 18 pages of detailed observations about influenza, including epidemiology, symptoms, and attempted treatments and vaccines. The treatment section is horribly incorrect, but the practices of epidemiology and publishing of findings was well-established, and the medical community was aware of the disease and current research. Another cited study showed that urine was found to contain highly increased leukocytes, as well as pneumococcal bacteria in influenza patients, likely confusing the symptoms of the influenza virus with other pneumonia-like diseases such as Streptococcus pneumoniae or Haemophilus influenzae, or maybe confusing sequelae (follow-up infections) with the main disease. Multiple case studies referenced discuss the significance of increased leukocyte counts in blood and urine, along with the observation of bacteria. Higher significance was placed on the leukocytes, since the specific bacteria was still debated.

Fourth, epidemiology.

Epidemiology was a field of study among the medical sciences before 1918, and there was no debate over the usefulness of tracking cases to determine solutions. The bulk of the excitement in the field was in sanitation and sewage, but airborne diseases and other tracking methods were still studied and published. The lay-person was knowledgeable about epidemiology and improvements in hygiene mostly because of the new sewer systems being installed, and developments in the prevention of scarlet fever, smallpox, and other communicable diseases. People were generally excited about these new developments, but confused and hesitant about constantly changing guidelines.

There are multiple fascinating discussions on air and sewage testing, isolation precautions, compulsory vaccination, and quarantine policies that could have been published today with very little editing. The chapter in the 1892 text on epidemiology gives a 20-step set of instructions to prevent the communication of disease that are meant to be published and handed out to people when an outbreak occurs. These steps include specific and simple instructions on how to isolate an infectious person, anything they may have come into contact with, disinfection, monitoring, and treatment of any remains.

It was still hotly debated. One expert stated "...There can be no doubt that that we may diminish its (infectious disease) incidence... we can make the soil upon which its seed is sown so inhospitable as to render it sterile." Another disagreed, stating that "quarantine was a tyranny, obstructing commerce and interfering with personal liberty."

References (All German references were translated to English. Germans were at the forefront of pathology research for much of the late 19th century.):

“Brock, Thomas D. (1988): Robert Koch. A Life in Medicine and Bacteriology.” European Journal of Protistology, vol. 25, no. 1, Sept. 1989, p. 85. DOI.org (Crossref), https://doi.org/10.1016/S0932-4739(89)80082-3.

Struck, E. (1881). Mittheilungen aus dem Kaiserlichen Gesundheitsamte: Bd. Mit 14 chromolithographischen Tafeln. Germany: Verlag von August Hirschwald.

Löffler, F., Gaffky, G. (1884). Mittheilungen aus dem Kaiserlichen Gesundheitsamte: Bd. Die Aetiologie der Tuberkulose. Germany: Norddeutsche Buchdruckerei und Verlaganstalt ; zu beziehen durch Louis Gerschel Verlagsbuchhandlung (G. Gossmann).

Annual of the Universal Medical Sciences. (1892). United States: F.A. Davis.

Zeitschrift für Pflanzenkrankheiten. (1903). Germany: E. Ulmer.