Is prayer for healing also not a type of "desire"? If we pray for someone with an illness, are we not trying to bypass the "karma" by virtue of which he/she is ill? Will it not have an impact on the "cause" and "effect" relationship of life?
It is true that path of yoga teaches that we should let go of all desires and all past karma. The reality of this process, and this is clearly stated in the Bhagavad Gita, is that we can’t do that all at once. It has to be a more gradual process.
Therefore, it is best to begin the process by letting go of desires which are obviously harmful to us. It’s OK to continue (for a while) to have desires which are helpful to you or others — such as sending a person healing energy and love.
Some good karma may be created in this way, but good karma has a way of alleviating bad karma, until we reach the point in our spiritual development where all karma is released.
Another important point about healing prayers and one which will help in the process of not creating more karma is to pray to be a channel for Divine Healing Energy to flow through you. Then you are NOT the do-er. God is the do-er and you are simply the instrument through which the Divine Healing Power can flow.
In offering healing prayers for someone, always pray that God’s will be done. If the person is not meant to be healed, then that’s God’s business, not yours. But you will have done no harm, to yourself or the person, with your humble prayers, especially if you do your best NOT to be attached to the outcome.
What you are sending to the person for whom you are praying is prana or "conscious cosmic energy." Because it is conscious, it knows exactly what to do better than you do. So not to worry about interfering with your own or another’s karma. It just doesn’t work that way, according to the great masters of yoga.
Healing prayers, when offered with the correct attitude, help everybody involved and can be a very beneficial part of your spiritual practices.