During the 1918 influenza pandemic, it's estimated between 20 and 50 million people died worldwide, killing around 2-3 percent of those who became infected.

Unlike the COVID-19 pandemic we're living through today, the age group most affected by the disease was young adults. This was possibly due to cytokine storms – where the immune system overreacts, sending out an abundance of immune cells, leading to inflammation and a buildup of fluid in the lungs. The young were counterintuitively more at risk of death due to their robust immune systems, which would trigger these storms in response.

Just like today, there was a rush to learn as much as possible about the disease, including how the virus was spread. Three doctors in Boston took unusual measures to attempt to find out just this, in a study that today would be seen as highly unethical as it is illegal.

Dr M.J. Rosenau and Lieutenant J.J. Keegan offered prisoners at the military prison on Deer Island in Boston Harbor a risky deal. If they were willing to participate in an experiment that would see them deliberately infected with the flu – and survived – they would be pardoned and released from their sometimes lengthy sentences.

The experiments would see some of the prisoners injected with infected lung tissue from sick or deceased patients, have infected tissue dropped in their eyes, and sprayed in the nose and mouth with infectious aerosols. Others would see mucus taken from critically ill patients and put it into the noses and throats of prisoners. In other parts of the trials, experimenters would take the blood of the sick and inject it into the healthy, to see if it was spread through infectious microorganisms in the blood.

300 prisoners saw this option and said "sign me up". Whether they truly knew the risks involved or were just so desperate for a pardon that they believed the risk to be worthwhile is unclear. Of these volunteers, 62 were selected to participate in the trials.

As well as the various fluid exchanges mentioned above, a further part of the experiments saw ten healthy prisoners taken into a hospital for patients who were dying of the disease. There, they were asked to stand over the sick and dying, lean over their faces and breathe in heavily while they exhaled. Just to be sure of exposure, the flu patients would cough into the face and mouths of the prisoners.

And yet, despite all this, not a single patient in the experiment died of the disease – they didn't even become ill. In fact, the only person who did become infected was a doctor involved in the experiments, who died shortly after contracting it. 

The likely explanation is that the prisoners had already been exposed to the disease some weeks earlier, gaining natural immunity to it prior to the experiment. Having already survived it once, they had now essentially been offered a free pardon – albeit with a strange few weeks of allowing dying people to cough in their faces, or eating the mucus of the deceased.