The factors which leave someone vulnerable to infidelity are present long before the unfaithful behavior begins. There is a build up of entitlement (I deserve to be happy; because my spouse is ignoring me, I deserve to get my needs met somewhere else; I am entitled to physical intimacy). There often is a growing sense of loneliness. Many spouses who stray do not know how to effectively communicate their needs. They operate under the false premise of "If you really love me, then you'd know how I feel without me having to tell you."
Many wayward spouses are conflict avoidant. They don't want to hurt their spouse by telling them how distant they feel, how unhappy they are, etc. What is actually going on is that the wayward spouse becomes anxious around bringing up difficult, conflictual subjects. So, he/she goes quiet, acts out, and hurts their spouse much more deeply in the long run.
An excellent book to help couples after an affair is called, Surviving an Affair by Harley and Chalmer. And of course, counseling is essential to help couples as well. Affairs simply don't happen when a marriage is in good shape. It's essential to look at what left the couple vulnerable to an affair to begin with. If you're in the Chicago area, check out one of our therapists at Heritage Professional Associates. Outside of the Chicago area, ask someone whom you trust for the name of a reputable mental health professional.