For over a century, scientists ran controlled experiments trying to prove that colds and flu spread from person to person. They exposed healthy volunteers to everything: nasal inoculations of mucus from sick people, contaminated handkerchiefs, sealed rooms full of coughs, direct face-to-face breathing. The results across all 203 experiments were not what most people would expect. Naturopath and health researcher Daniel Roytas spent years systematically reviewing the primary scientific literature on cold and flu transmission and assembled one of the most comprehensive summaries of this evidence available in a single work.
- 203 controlled human transmission experiments conducted between 1906 and 2020
- The most common single result across all experiments was zero illness
- The US Navy's 1918 experiments failed to infect a single volunteer across 25 separate protocols
- Non-infectious causes including air quality, temperature, and expectation of illness can produce the same symptoms attributed to viruses
- The scientific criteria designed to prove germ causation could not be satisfied by the researchers who created them
What 203 experiments actually produced
Between 1906 and 2020, researchers ran more than 200 controlled experiments attempting to transmit colds or influenza from sick people to healthy volunteers. The mean positive rate across all studies was 32%. After accounting for control groups that produced illness with no infectious material at all, the adjusted rate drops to around 28%. Four outlier experiments accounted for nearly half of all positive cases. Remove those, and the overall rate falls to 22%. Seventy-three experiments, more than a third of the total, produced zero cases.
These were not small, poorly resourced studies. The US Navy series during the 1918 Spanish flu pandemic used 62 healthy volunteers in excellent physical condition. Across eight experiments at Deer Island in Boston, none became ill. This included one protocol where sick men breathed and coughed directly into the faces of healthy sailors for up to 50 minutes. The same result was replicated at Angel Island and Gallups Island. Across all 25 protocols in that series, not one volunteer was infected.
When the results are read without the assumption that contagion is already proven, they raise a straightforward question: if transmission works as reliably as we are taught, why did these experiments so consistently fail to demonstrate it?
The problem with the positive results
Even the experiments that did produce illness are difficult to interpret. Almost none of the 203 studies used positive controls, random sampling, or adequate blinding. Without these design features, a participant who develops symptoms after inoculation could have done so from the procedure itself, from a chemical reaction to the non-viral components of the fluid, from coincidental natural illness, or from the expectation of becoming sick.
That last possibility is not theoretical. In one experiment, a participant was told he had received mucus from a sick person. He developed a severe cold by that evening. The following day, he was told the truth: he had received plain saline. His symptoms resolved within an hour. This is the nocebo effect in action. It is the mirror image of the placebo effect. Negative expectation alone can produce real, measurable symptoms, and removing the expectation can reverse them.
In another experiment, a researcher inoculated 43 healthy volunteers with plain saline and 19% developed a common cold. That was a higher illness rate than some of his active inoculation groups.
When your airways become their own problem
The respiratory tract is lined with a thin layer of fluid called airway surface liquid (ASL). Its normal pH sits between 6.9 and 7.1, slightly acidic. When that pH drops below 6.9, three things happen at once: cells on the airway surface are damaged, mucus becomes thicker and harder to move, and the sweeping mechanism that clears inhaled debris from the lungs begins to fail.
If pH drops below 6.7 and stays there for more than 24 hours, respiratory cells begin to die and detach from the airway wall. In the medical literature, cell death and detachment in the respiratory tract are listed as hallmarks of viral respiratory infection. But they also occur in response to ASL acidification, with no pathogen involved at all.
Three documented causes lower ASL pH: inhaled particulate matter from air pollution, sulfur dioxide (which is four to six times higher in winter due to increased fossil fuel combustion), and dietary imbalance. Each has a plausible seasonal pattern. None requires a virus.
Why the foundations of germ theory matter here
The scientific criteria for proving that a specific germ causes a specific disease were created in the 19th and early 20th centuries. Koch's postulates set the standard for bacteria. Rivers' postulates adapted the standard for viruses. Both frameworks require demonstrating that the purified agent, when introduced into a healthy host, reliably reproduces the disease.
Neither Koch nor Rivers could satisfy their own criteria. Koch could not fulfil his postulates in relation to cholera. Rivers could not satisfy his for viruses. Rather than treating these failures as disqualifying, later researchers introduced the concept of asymptomatic infection to explain cases where the pathogen was present but the disease did not appear. This moved the goalposts: a theory that cannot be falsified by negative results is not behaving like a scientific theory.
Alternative causes with documented mechanisms
Absolute humidity reliably predicts influenza outbreaks in temperate climates. For every 0.5 g per cubic metre drop below a threshold of 8 to 12 g per cubic metre, influenza risk increases by around 58%. Cold temperature exposure has been associated with increased illness rates in both epidemiological data and controlled experiments. In one controlled experiment, participants who immersed their feet in cold water developed colds at more than twice the rate of controls. During the First World War, two experiments involving a combined 8,000 soldiers found that those sent into cold, wet conditions fell ill at four times the rate of those kept in warm barracks.
None of this means transmission is impossible. It means the evidence for it has not met the standard we would apply to any other causal claim, and the non-infectious explanations deserve serious investigation rather than dismissal.
How to use this for yourself
Understanding that expectation of illness can trigger real symptoms is not just a curiosity. It changes how you might respond when you feel something coming on. Catastrophising, assuming the worst, and telling yourself you are about to get sick may not be neutral acts. The reverse is also documented: calm, expectation of recovery, and removal of the nocebo framing have been associated with faster resolution of symptoms in controlled settings.
Similarly, if cold and flu symptoms can be triggered by ASL acidification from air quality and diet, then optimising both becomes a practical strategy for respiratory resilience, not just general wellness. These are levers within your control.
Go deeper with the full document
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