Scientists say there is growing evidence that a virus may play a role in the development of breast cancer.
The tests threw up geographical differences
Tests by researchers in the United States have found signs of a virus called MMTV in tissue taken from women with the disease.
But writing in the journal Cancer, they said there were geographical variations in the numbers testing for the virus.
The UK charity Breast Cancer Care said more research is needed to determine if there is a link.
Dr Paul Levine and colleagues from The George Washington University School of Public Health carried out tests on tissue samples taken from breast cancer patients in Europe, North and South America and North Africa.
They found that 74% of the samples taken from patients in Tunisia showed signs of MMTV.
This compared to 42% of the samples from Australia, 38% of those from Italy, 36% of those from the United States and 31% of those from Argentina.
Tests on samples from women from Vietnam found that less than 1% showed signs of the virus.
MMTV or mouse mammary tumour virus is known to cause breast cancer in mice. Previous studies have found signs of the virus in breast cancer tissue taken from women.
The researchers said animal studies have found high levels of this virus in aggressive cancer tumours.
But they said: "Whether this can be extrapolated to humans remains to be demonstrated."
The researchers suggested the geographical variations may be directly related to MMTV in mice.
"The geographic differences were compatible with studies of MMTV in wild mice," they said.
Helen Graham, a breast health nurse specialist at Breast Cancer Care said: "This study adds to existing research suggesting there may be a link between the MMTV virus and the development of breast cancer.
"The study had a very small sample and it is clear that more extensive research is needed into the possible link between MMTV and breast cancer.
"Many of the women Breast Cancer Care talk to are anxious to understand the causes of breast cancer but it is very important for all to remember that the single most significant risk factor for breast cancer is age.
"Therefore, every woman should be breast aware throughout her adult life."