Two human challenge trials were done during the 1918 Spanish flu by two independent groups of doctors in Boston and San Francisco with 62 and 50 healthy volunteers, respectively. Regardless of how many aggressive means were taken to try to infect people (even dropping mucus or bodily fluids from flu patients into the healthy volunteers' eyes, noses, or throats), none of the participants became infected. During the COVID-19 pandemic, a SARS-CoV-2 human challenge study published in Nature in 2022 found that of the 36 healthy volunteers inoculated with the SARS-CoV-2 virus intranasally, only a little more than half became infected with mild symptoms, and the other half remained uninfected. Two were excluded from the per-protocol analysis, so the experiment went on with 34 participants.