The researchers examined 87,000 cell samples from 12 people, split between those who had the virus but no cancer, those who had the virus and the cancer, and those without the virus.
T-cells in people who had the virus were over-reactive, producing large amounts of proteins involved in proliferating and avoiding the parts of the immune system that hunt down cancer cells.
The team believe this change in activity makes the cells more susceptible to DNA damage that can turn them fully cancerous.
The resulting cancer, adult T-cell leukaemia, is highly resistant to treatments and has a high relapse rate following chemotherapy.
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