Cheating on a Sick Spouse Illness and its fallout open the door to adultery, experts say.

Aug. 12, 2008  -- John McCain. Then Newt Gingrich. And now John Edwards.

Edwards' betrayal of his wife at her most vulnerable moment -- as a cancer patient -- is more common than conventional wisdom suggests, according to infidelity experts.

A mistress of former House Speaker Gingrich told Vanity Fair they had their tryst as his first wife recovered from uterine cancer surgery in the 1980s.

While McCain was married to his first wife, who was disabled from a car accident, he was "aggressively courting a 25-year-old woman who was as beautiful as she was rich," according to a New York Times colunmist's account of the 1970s incident that predated McCain's election to the Senate.

Both McCain and Gingrich have admitted to the affairs.

"It's not that uncommon for the spouse of a seriously ill person to commit adultery," said Anthony DeLorenzo, who, with his wife, founded infidelity.com. "The healthy spouse often feels guilty, lonely and helpless about the illness, and that combination can make a spouse more vulnerable to having an affair."

Sickness frequently interferes with or eliminates sex from a relationship, making a healthy spouse more vulnerable to advances or situations that lead to sex outside of marriage, the New Jersey private detective told ABCNews.com.

Blogs this week have been less sympathetic, calling Edwards and his extramarital sex with videographer Rielle Hunter "very low" and "depraved."

"Edwards claimed, while engaging in the affair, that Elizabeth was in remission," read one post on the site, the World According to Matt. "In short he is a cad."

Relationship experts like DeLorenzo, who counsels wronged spouses on his Web site, say about 60 percent of men and 40 percent of women stray during the course of a marriage.

"I know Edwards spoke of his narcissism as a factor, but far more than that had to be going on," said DeLorenzo, who has handled many cases of cheating.

And it's not just the men who are going astray. One middle-aged man who was confined to a hospice-like setting suspected his wife was having an affair when he got no answer to his telephone calls to her on Friday and Saturday nights, DeLorenzo said.

"His wife took care of him three or four times a week, but she was in her 50s and was still a young person," DeLorenzo said. After receiving the report confirming the affair, the man eventually accepted his wife's dalliances.

But according to Thomas Nagy, who is on the clinical faculty at Stanford School of Medicine and works with couples dealing with cancer, that and other chronic diseases can have an especially devastating effect on a marriage.

Depression, anxiety and medications can take a toll on the spouse who is the patient.

"It hugely impacts on moods and emotional intimacy -- everything," Nagy said. "Mental health treatment is extremely important. Most are not in spousal support or marriage counseling."

He advocates for mandatory marital counseling whenever a partner faces a serious illness.

In Cancer, Illness 'Trumps Everything'

"The mental health needs of a marriage aren't paid attention to because the medical illness trumps everything," Nagy said.

The Edwards couple -- at the center of political power -- got the "one-two punch," according to Nagy. "His wife is having cancer in this really high-profile fast life."

But women who have been wronged have less sympathy for the senator and more for his wife.

"My heart went out to her and for the embarrassment she is suffering," said one New Jersey woman, whose husband has been having an illicit affair for the past seven years, but won't admit to it. "I just wish I had an answer."

The 53-year-old, who works in marketing, said her husband of 33 years recently had quadruple bypass surgery after a clandestine weekend with the other woman. As his wife, she stood by his side at the hospital.

"I was miserable," said the woman, who didn't want to be identified. "He was at death's door. I am no angel, but I didn't want to make the situation worse or add to the stress. I was trying to make things easier for him."

In her time of need -- during eye surgery and later for a cancer test -- he wasn't there. "He couldn't take the time off from work," she said. "He just has excuses: That's his nature.

"I was there every day for him and sat there thinking he would go through a big metamorphosis and turn things around," she said. "I don't know why I am still here. I told the kids, 'He's in your hands now.'"

Michigan therapist Bob Huizenga anonymously coaches clients on his Web site and in his book of the same name, "Break Free From the Affair." He said women who have been wronged have a wide range of responses.

"Some cave in and feel victimized and others say, 'Screw it,' and fight and win," he told ABCNews.com. "It's utterly devastating."

One woman sought help from Huizenga when her husband turned to a girlfriend after her breast cancer metastasized. "Not only are you losing your body parts and vitality, but also losing what you perceive to be your dreams, your family and your social status," he said.

"It's worse than someone dying."

Many couples don't survive the betrayal, but power couples like the Edwardses often save their public face and work things out.

'High-Ups' Struggle With Infidelity

Some of Huizenga's recent online patients have been "high up" in Washington politics, he said.

"Spouses are having affairs and have nowhere to go," he said. "It's a safe relationship talking to me anonymously. They wonder how to deal with the mistress or with the cheating spouse."

Like Edwards, they worry about their reputations and being exposed, according to Huizenga.

And when a woman has cancer, she has to consider the financial repercussions of leaving the spouse -- and his insurance -- behind. And, as in the Edwards' case, there are sometimes young children to consider.

"It's very difficult for someone like [Elizabeth Edwards] to leave in this situation," he said. "She has a high investment in maintaining that relationship. She may submerge a great deal. Political wives do. They have an investment in the political arena as well."

But, he said, in all cases it's hard to take the high road in infidelity cases. "I am not sure what the word forgiveness means, but they never forget," he said. "It's always there."