Next to lung cancer, breast cancer kills more women in the United States than any other cancer. It's the leading cause of death in women between the ages of 40 and 44 and one is 13 women will have it sooner or later.

"Breast cancer is the third most common cancer in the world today, despite the fact that it is confined almost entirely to the female sex. Breast cancer incidence rates are rising in several developing countries, and it is already the most frequent female cancer in many," revealed Dr. Corazon A. Ngelangel, professor, Clinical Epidemiology Unit, College of Medicine, and a consultant at the Medical Oncology Section of the University of the Philippines - Philippine General Hospital.

The exact cause of breast cancer is unknown. But the risk of developing the disease increases after the age of 35, especially in those with a family history of the disease. Breast cancer is also more common in obese women and in those who have never had children or who had a child for the first time after the age of 30.

Those who begin menstruation early and women who had late menopause are also at risk for developing the disease. The same is true for women who previously had breast cancer. The odds are high that these women will have the disease in the other breast too.

Unknown to many, breast cancer can affect men but not as frequently as women. Less than one percent of cases occur in males, usually at middle age or older.

Alcohol is believed to be a factor in male breast cancer. Other high-risk groups include those who are exposed to dust, gasoline, grease, carbon monoxide, and radiation. Men with gynecomastia, the enlargement of the breast that follows exposure to estrogen and alcoholic cirrhosis (in which the liver has been destroyed by years of heavy drinking) are likewise susceptible to the disease.

"Although breast cancer can occur in young boys (the youngest reported was five years old), the incidence generally rises with age. The majority of patients are more than 60 years old. Earlier cancer denotes a strong family history, manifested by the presence of many female relatives with breast cancer," said Dr. David Y. Dy, a general surgeon and surgical oncologist at St. Luke's Medical Center in the Philippines.

Surprisingly, male breast cancer appears to be less common in married men and in those with many children. Like women, most cases have been observed in the left breast although it is not clear why.

"About half of all breast cancers develop in the upper outer portion of the breast, the part of the breast closest to the underarm. The second most common site is the area surrounding the nipple, where about 18 percent of breast cancers are found," said Marrion Morra, assistant director of the Yale Comprehensive Cancer Center at Yale University School of Medicine in Connecticut, and Eve Potts in "Choices: Realistic Alternatives in Cancer Treatment."

Still, majority of breast cancer cases occur in women. Ngelangel believes the presence of the female hormone estrogen may have something to do with the disease.

"The most important risk factor for developing breast cancer is sex. Being female puts you at great risk. The reason for this is estrogen. Women have more of this hormone than men which probably explains why breast cancer is common among females," Ngelangel said. (Next: Breast cancer myths.)