Medical mystery surrounds 7 sisters diagnosed with breast cancer

BOSTON -- A family in Boston is at the center of a medical mystery after the seventh of seven sisters was recently diagnosed with breast cancer.

The siblings have been battling the disease successfully since 1995.

There is no history of the disease in their immediate family and genetic testing showed they do not carry the BRCA genes associated with an increased risk of breast cancer.

The doctor who has treated the sisters says there could still be a genetic or environmental link, but she's exploring another theory.

"It's something unusual, maybe it's a virus," suggests the doctor.

The sisters' doctor has connected them with a group investigating the role viruses may play in causing breast cancer.

The researchers plan to test the women.