It has long been known that regular, moderate exercise is one of the best things you can do for your health, with studies showing that trained athletes have increased levels of natural killer cells that fight infection. But as anyone who practises endurance sports probably knows, hard training for an event like a cycle race or a marathon run can leave you with a dandy head cold or other upper respiratory tract infection (URTI).

At the moment I’m trying to fight off a suspicious soreness in my throat, a warning sign that a URTI may be on the way. I wondered: what is it about working out and sweating heavily that makes you more vulnerable to illnesses?

David Nieman, a researcher, tracked 2,311 participants in the 1987 Los Angeles marathon and came up with some startling results: during the week following the race, 12.9 per cent of runners reported being sick, compared with 2.2 per cent of control runners who did not take part in the race that year.