Unforgiveness can not only be destructive to a person's spiritual health, but also his or her physical well-being.
As doctors learn more about these effects, many are beginning to integrate "forgiveness therapy" into certain treatments.
Jayne Valseca said if she hadn't learned to forgive, she may not even be alive today.
She and her husband Eduardo were in Mexico when he was kidnapped and tortured. Valseca eventually negotiated his release, yet she had trouble moving on from the ordeal. She describes the experience in her book, We Have Your Husband.
"I knew that the stress was taking a toll on my immune system," Valseca recalled.
The Physical Toll
After eight months, her husband was released, but she soon found out she had another battle to fight.
"I was almost not surprised, yet completely devastated to hear the words, 'You have stage four breast cancer,'" she said.
Valseca prayed for healing even though doctors gave her a death sentence.
"I did collect more information, and in one of the books I read, I heard about Cancer Treatment Centers of America and I thought, 'Wow, this sounds like a fantastic place,'" she said.
Valseca added "forgiveness therapy" to her cancer treatment with Dr. Michael Barry, a pastor and author of the book The Forgiveness Project.
"Harboring these negative emotions, this anger and hatred, creates a state of chronic anxiety," Barry explained.
"Chronic anxiety very predictably produces excess adrenaline and cortisol, which deplete the production of natural killer cells which is your body's foot soldier in the fight against cancer," he said.
Power of Forgiveness
Barry's research on cancer patients revealed about 61 percent had trouble forgiving. More than half had a severe problem. Valseca said she had been consumed with revenge against her husband's kidnappers.
Forgiveness therapy begins with putting to rest three myths -- forgiveness is not reconciliation, forgiveness doesn't condone bad behavior, and forgiveness doesn't stop the pursuit of justice.
Valseca's path to forgiveness included writing a letter to her enemies, praying for them, and finding empathy for them.
"I saw the kidnappers as babies. And I took them one by one... in an imaginary scenario that I had created, going through things that I thought they could probably have gone through in order to get to where they got to do the things that they do, and to do what they did to me and to my family," she explained.
"And it was then that I felt this huge sense of relief," she said. "It was as if this huge ton of bricks, this weight of the world, was instantly lifted from my shoulders."
"When a person forgives from the heart, which is the gold standard we see in Matthew 18, we find that they are able to find a sense of peacefulness," Barry added.
"Quite often our patients refer to that as a feeling of lightness," he said. "We don't realize what a burden anger and hatred is until we let it go."
Fighting the 'Disease'
In medical literature, unforgiveness is classified as a disease, which is defined as "some process that overwhelms normal function."
"It's important to treat emotional wounds or disorders because they really can hinder someone's reactions to the treatments -- even someone's willingness to pursue treatment," said Dr. Steven Standiford, chief of surgery at the Cancer Treatment Centers of America.
The Valsecas have moved on from their trauma and unforgiveness and are keeping a positive outlook.
"There is nothing I can do with my mind thinking of negativity or thinking all the bad things they did," Eduardo said.
The couple, along with Dr. Barry, were honored by President Barack Obama at the Hispanic Prayer Breakfast this year for their living testimony of the power of forgiveness.
*Original broadcast June 8, 2011.