For decades, cancer has been thought of as a purely human disease – rogue cells multiplying out of control, with no room for anything else in the picture. But a growing body of research suggests that isn’t quite right. Some tumours, it turns out, come with company: communities of bacteria, viruses and fungi living on, between and even inside the cancer cells themselves.

The trouble is that nobody has been entirely sure which cancers actually have this so-called microbiome, and which don’t. The field has been dogged by contradictory claims, competing methods and – in one particularly damaging case – a retraction, after results from a high-profile study could not be replicated.

Since then, the field has been left without a clear way forward. Every research group has used its own methods and level of rigour, and there has been no agreed-upon benchmark to check new findings against. That matters because the stakes are high.

If microbes really are helping some cancers grow, resist treatment or spread, they could become new targets for screening and drug development. But chasing signals that turn out to be false wastes time, money and precious patient samples.

Our team set out to settle the question properly, using the largest collection of cancer genetic data in the world –........

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