When the former England and Newcastle manager Kevin Keegan recently revealed that he had stage 4 cancer, the footballing world responded with an overwhelming show of support. But hidden within his story is a surprising lesson about how cancer is often discovered – not through symptoms, but by chance.

Keegan was in a car crash just weeks before his diagnosis. Studies have found that people who suffer car crash injuries are more likely to be diagnosed with cancer than similar people who haven’t.

Most experts agree that car crash injuries don’t actually cause cancer. So what explains the link?

One possibility is a shared underlying cause. People who drive a lot, simply by spending more time on the road, are more likely to have accidents. They may also lead more sedentary lives, a factor linked to higher cancer rates. Frequent drivers may be more likely to be overweight or to spend long hours in the sun (exposed to harmful UV radiation), both of which raise cancer risk. Sleep deprivation is another candidate: it raises the risk of both crashes and cancer.

To resolve the conundrum, we have to consider how we detect cancer. Typically, we diagnose cancer in people who undergo some type of medical examination – either because their cancer has caused them to feel unwell or for some other reason.

When people........

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