In the late 1980s, 15 healthy people moved into new apartments in Salisbury, UK. On their third day, each was asked to snort a nostrilful of solution containing a coronavirus — one of several viruses that causes the common cold. Then the volunteers spent three weeks quarantined at the Common Cold Unit, part of the Medical Research Council, where researchers monitored a range of symptoms. Some study participants likened stays at the Common Cold Unit to a vacation — if so, it was a holiday complete with blood draws and nasal washes.

About a year later, 14 of the volunteers came back to do it again. This time, researchers were keen to know if the participants’ exposure to the virus had made them immune. The answer: sort of. Although they showed no symptoms, analyses revealed that nearly all of them became infected before their immune systems could launch an effective defence.

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