If I had a platform to teach evangelicals about sin, the first thing that I would tell them is that sin is not a disease. At least, it is not like a virus or a bacterial infection. Unfortunately, it appears that this is how we tend to think of sin, and even more unfortunate is the tendency to think that sin can be 'cured' here by prayer, piety, or will power.
This type of thinking is detrimental to Christians, especially people young in the faith who do not know any better. The immature tend to think of sin in terms of things we do: like cussing, looking at pornography, sleeping with one's girlfriend, or drinking too much beer on the weekend. The cure for these 'sins' then, is to simply stop doing what you used to do that the Bible calls sinful. Thus, the young Christian thinks that he is becoming better by stopping his or her sinful behavior, and because of this, they may declare that they have been cured.
In the zeal that comes from first understanding the gospel of Christ, one's resolve to be obedient can indeed squash down old sinful impulses. The problem comes when those old impulses arise again. The believer may begin to think that something is wrong with them, which is correct, but they may blame it on the wrong thing. They may think that God has suddenly abandoned them or that they have become less faithful. Neither may be the case. They may simply be learning what having a sin-nature really means.
If sin were to be compared with a disease, it would have to be compared with a genetic one that is currently incurable but is treatable. Let's take high blood pressure as an example. High blood pressure can be genetic, and the effects of high blood pressure can be aggravated by poor eating and exercise. In the case of conversion, a young believer begins to 'live healthy', forsaking harmful behaviors of the past, and so the symptoms of sin lessen. Over the course of time they find, despite their good diet and exercise, they are still having high blood pressure. The only way to treat this is by medicine. In our case, the medicine is grace.
I worry when people begin to say that homosexuality and other sins may be 'cured' by prayer or some other form of piety. This is tantamount to saying that lust may be cured. Our sin is an inherited disposition. It is no simple disease. The symptoms of sin will manifest despite the fact that the Christian is living faithfully. God allows these impulses to continue so that we will learn to take our medicine, that is the grace that is offered us through the gospel of Jesus Christ.
I am a pastor serving in my hometown of Albertville, Alabama. The greatest evidence of God's grace in my life are my wife, son, and daughter. One look at me and then my wife will tell you that her "yes" was a modern day miracle. Otherwise, I am almost completely mundane.