The 1918 Influenza Transmission Experiments: What They Found and What They Do Not Prove

Short answer: Milton Rosenau’s 1918-1919 influenza experiments are real and fascinating. His team failed to reproduce influenza in healthy volunteers despite several aggressive exposure methods. But that result does not show that influenza is non-contagious or that influenza viruses are laboratory artifacts. It shows that an early experiment, performed before the influenza virus itself had been identified, failed under its particular conditions.

I originally wrote about these experiments because the result is genuinely surprising. After revisiting the historical record and comparing it with later controlled human infection studies, I think the most useful question is not “why was this buried?” but why did this particular experiment fail while later challenge experiments succeeded?

What Rosenau actually did

A historical review of the 1918 pandemic describes U.S. Navy experiments led by Milton Rosenau at Gallops Island. Volunteers received respiratory secretions, blood, bacterial preparations, and in one protocol close face-to-face exposure to symptomatic patients. None developed influenza. Importantly, Rosenau himself warned against drawing a broad conclusion from the negative result.

The experiments were conducted before influenza A virus was identified in humans. Investigators could not quantify the infectious viral dose, confirm the viral strain in each donor, measure neutralizing antibodies, or standardize aerosol particle size and deposition in the respiratory tract. Those limitations matter because transmission depends on the infectious dose, route, timing of shedding, host immunity, and environmental conditions.

Modern human challenge studies give a different result

Controlled human influenza infection has subsequently been reproduced many times. A 2025-2026 modern aerosol challenge study exposed healthy adults to quantified influenza A/H3N2. At higher doses, infection occurred in 50% to 75% of participants, with respiratory symptoms, viral shedding, and immune responses. Earlier aerosol challenge experiments also produced influenza infection in volunteers.

This does not make the Rosenau result meaningless. Quite the opposite: it is a useful reminder that failure to transmit a disease in one experimental setup does not identify which part of the setup failed. A negative challenge can reflect dose, route, donor selection, immunity, pathogen identification, sampling, or other variables.

Infection is not the same thing as symptoms

Another point from my original article remains important with a correction. A positive molecular test and clinical illness are not identical concepts. A person can be infected with few or no symptoms, and a nucleic-acid test does not by itself measure severity or prove that a person is currently contagious. But this distinction does not mean that laboratory-confirmed infection is fictitious. Modern challenge studies can follow the entire sequence: known viral exposure, viral replication and shedding, symptoms, and antibody response.

Where stress fits

Expectation and stress can alter symptom perception, but they are not an alternative explanation for influenza infection. Host biology does matter. Immunity, prior exposure, sleep, age, stress, and other factors can influence whether exposure leads to symptomatic disease. That is very different from saying that symptoms are produced primarily by suggestion.

Bottom line

The Rosenau experiments deserve to be remembered because they were unusually direct and produced an unexpected negative result. The strongest conclusion is narrow: the investigators could not transmit influenza with the methods they used in 1918-1919. Modern controlled infection research shows that influenza virus can reproducibly infect humans, including through aerosol exposure. The historical puzzle is therefore about experimental conditions, not evidence that influenza contagion is imaginary.

Sources

• The State of Science, Microbiology, and Vaccines Circa 1918, historical review including the Rosenau experiments.
• Feasibility and Safety of Aerosolized Influenza Virus Challenge in Humans Using Two Modern Delivery Systems, Journal of Infectious Diseases.
• Airborne transmission of respiratory viruses including SARS-CoV-2, review of human influenza aerosol challenge evidence.