Joan Lunden, former co-host of "Good Morning America," announced on the program Tuesday that she has been diagnosed with breast cancer, and has begun treatment. Lunden, 63, told Robin Roberts -- who underwent treatment for a bone marrow disorder, myelodysplastic syndrome, that had been related to her cancer -- that an earlier mammogram had been negative, but that a malignancy was later detected by an ultrasound.

"I heard those those words that every woman fears and never wants to hear, 'you have breast cancer,' " she told Roberts, while adding that since then she has "learned everything you can about this and [has gone into] warrior mode." She said the cancer is "of the more aggressive kind" and that treatment will include surgery, chemotherapy and radiation.

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Lunden, a mother of five, including three daughters from her first marriage, also posted in her blog, "I have already begun my chemotherapy and I am blessed to have my husband Jeff [Konigsberg] and my three older daughters with me every step of the way."

An author of eight books -- most on health and healthy lifestyles -- Lunden was co-host of "GMA" from 1980 through 1987. Last November, "GMA" news anchor Amy Robach also announced that she had been diagnosed with breast cancer following an on-air mammogram. A few days later she underwent a bilateral mastectomy.