The human immune system keeps a record of pathogens it has met before, in the form of antibodies that fight against them and then stick around for life. By testing for these antibodies, scientists can get a snapshot of which flu viruses you have had, what that rhinovirus was that breezed through you last fall, even whether you had a respiratory syncytial virus as a child. Even if an infection never made you sick, it would still be picked up by this diagnostic method, called serological testing. “We’re all like little recorders,” keeping track of viruses without realising it, Dr. Mina said. This type of readout from the immune system is different from a test that looks for an active viral infection. The immune system starts to produce antibodies one to two weeks after an infection begins, so serology is retrospective, looking back at what you have caught. Also, closely related viruses may produce similar responses, provoking antibodies that bind to the same kinds of viral proteins. That means carefully designed assays are needed to distinguish between different coronaviruses, for example.