One of Buddha's main teaching is on non-attachment. And another virtue that Buddha taught is Forgiveness.
If one puts the two together, then forgiveness comes a lot easier. When someone hurts us, abuses us, we get angry, holds a grudge, plans revenge to hurt that person back. We are so attached to our egos that we need to 'get justice' , 'take revenge , make that person suffer '. That is all the ego talking . At this point , we are feeding our ego more and we will never even want to talk about forgiving .
Being infected with a fatal contagious disease is absolutely devastating , but we need to understand that it can be our karma to catch this disease in the first place , that everything is impermanent , we start our dying process on the very moment we are born. If we have the karma to have this deadly disease in our present lifetime, are not not creating more bad karma by not forgiving the person who infected us.
Forgiveness is a powerful tool to release oneself of the anger that is contained within us, and forgiving oneself releases the guilt that will eat into us and eventually make us more sick at all levels.