Only family history, age and radiation could be bigger causes of the cancer, say researchers at the University of California at Berkeley.
In a study of 239 women with breast cancer, the researchers discovered the virus in 59 per cent of the sample. The researchers also discovered that the virus was in 38 per cent of beef products and 84 per cent of cows' milk they tested.
The virus has also been found in the breast tissue of 29 per cent of healthy women who were checked, but this could be explained by the long latency period of breast cancer.
Around 90 per cent of cases of breast cancer are inexplicable, but the BLV could be another major cause, and this is borne out by the higher rate of the disease in countries where milk consumption is also greater.
Cooking and pasteurisation kills off the virus, but it can remain active in undercooked meat or unpasteurised milk.
(Source: PLoS One, 2015; 10: e0134304)