Why might one person get ill while someone else is fine?

For starters, our immune systems are all different – we’d have been wiped out as a species by now if they weren’t. This is partly down to genetics – it’s estimated that around 20% to 30% of your immune system is inherited from your parents. As for the rest? Dr Macciochi struggles to fit all the things that influence your immune system onto one lecture slide. “It’s everything from how good your gut health is to how many infections you’ve been exposed to; your diet, sleep and exercise and how much fat sits around your middle,” she says. This lack of understanding of how each of our immune responses will react to the same virus is part of why social distancing was critical in the pandemic. However, Dr E John Wherry, director of the Institute for Immunology at the University of Pennsylvania, predicts that, within a decade or so, we could get to the point where your GP is able to give you a personalised “immune score”.