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The link between prostate cancer and an infection is perhaps the weakest of all those here, but if confirmed it could lead to better screening techniques. The existing screening method 鈥� testing men鈥檚 blood for prostate-specific antigen (PSA) 鈥� is notoriously unreliable, so some men undergo unnecessary surgery that can cause impotence and incontinence.

In 2006, a team led by Joseph DeRisi, a biochemist at the Howard Hughes Medical Institute in San Francisco, examined prostate cancers for viruses and found that a few of the samples contained a new virus closely related to xenotropic murine leukaemia virus (XMRV), which is known to cause cancer in mice.

At this stage few were convinced of the link as the virus was not found in the cancerous cells themselves but in the surrounding tissue. 鈥淭he results were inconclusive,鈥� says Greg Towers, a virologist at University College London.

The case against XMRV has just been bolstered, however, by a study published last month by pathologist Ila Singh at the University of Utah in Salt Lake City. Her team found XMRV in prostate cancer cells themselves 鈥� in 29 per cent of cancerous cells compared with 6 per cent of healthy ones. And the more aggressive the tumour, the more likely they were to find the virus lurking within it ().

Both groups believe that while the virus probably originated in a mouse, it can now spread from person to person, though there are no clues yet as to how. Nor is it clear how the virus causes cancer, although other retroviruses are known to do so by inserting themselves into the host cell鈥檚 DNA and disrupting the genes that normally control cell division.

Singh now plans to develop a blood test for XMRV, which could be used as an adjunct to PSA testing, or perhaps even to identify fast-growing prostate cancers.

Read more: Six diseases you never knew you could catch