Carol Witcher says she knows it sounds crazy, but she swears that her dog, Floyd Henry, discovered the cancer in her breast in 2008.
"When he sniffed me, he kind of turned back and really pushed into my right breast, real hard," she said. "He started sniffing, sniffing, sniffing, sniffing."
It took four days of nudging and nipping by the 8-year-old boxer before Witcher went to a doctor.
"He pushed real hard for one shot. ... Then he looked at me straight in the face, took his right foot and began to paw my right breast. And I thought, 'This is not good,'" she said. "I knew instantly that there was an issue."
Witcher's stage-three cancer required surgery, chemotherapy and radiation.
"Her type of cancer was rather large in her breast," said Dr. Sheryl Gabram-Mendola, a breast surgical oncologist at Winship Cancer Institute of Emory University. "I absolutely believe that the dog saved Miss Witcher's life."
Gabram-Mendola has been studying the breath of cancer patients. She said cancer causes the body to release certain organic compounds that dogs can smell but people cannot. Gabram-Mendola and her team have developed a test in which they look for more than 300 molecules in the breath.
"Our model predicted in over 75 percent of the time correctly which patients did have breast cancer and which ones did not," Gabram-Mendola said. When Witcher breathed into the tube, the test confirmed that she was sick.
"You could potentially go to a physician's office, blow in the bottle and ultimately have a direct read system where we would know in the office: Are you showing some of those indicators [that] something's happening in your body?" Gabram-Mendola said.
Breath Holds Clues and Details
Experts say the breath holds clues that doctors are just learning to decipher.
In January, a study published in the British journal Gut said that a specially-trained 8-year-old black Labrador retriever named Marine had detected colorectal cancer 91 percent of the time when sniffing patients' breath, and 97 percent of the time when sniffing stool.
In August 2010, a terrier named Kiko bit off his owner's big toe, alerting the 48-year-old man to his undiagnosed type 2 diabetes.
It's estimated that a dog's sense of smell is up to a million times better than that of a human, depending on the breed. Dogs have also reportedly sniffed out skin, bladder, lung and ovarian cancers.
"Dogs smell different things and they understand different things," said Charlene Bayer, a principal research scientist at Georgia Tech Research Institute. "They don't necessarily know what's wrong, but they know that there's something that's not normal, that you don't smell the way you normally do."
Dogs Trained to Find Cancer in Urine
In an ongoing study in England, researchers are training a handful of dogs to identify the urine samples of patients with cancer. In a 2004 study, they correctly detected bladder cancer 22 times out of 54 tries.
Carol Witcher has been free of cancer for two years. "I had rescued him when he was about seven months old," she said of Floyd Henry. "He, in turn, rescued me."
ABC News' Katie Moisse and Courtney Hutchison contributed to this story.