The Bible says, "Do unto others as you will have them to do unto you."
"A Bitter Disease Called Unforgiveness" calls to you to check for bitterness in your heart, toward anyone about anything, and let it go. Bitterness will take a toll on you as though parasites are eating away every organ in your body, at a slow pace.
When author JoAnne Smith Hooks battled cancer thirty years ago, she lay in a hospice unit waiting to die, and finally came to a conclusion that the disease stemmed only from bitterness. Molested at the age of thirteen and turning to alcoholism by sixteen years of age, JoAnne knew her pain was the root of her illness, and if she didn't kill that root, it would definitely kill her. So she let it go and forgave.
She seeks to encourage you to do the same, strip yourself of all the bitter roots and let go of the pain. If you don't the "unforgiveness" will steal your joy, destroy your peace, and override your love.