If hurting others tends to lead to illness in the next lifetime, is illness in this lifetime always the result of past karma?
As with every other experience of pain, illness can come from a wide variety of karmic factors, past and present. You’ve already noticed this yourself: When you intentionally stick your finger in a fire, the resulting pain doesn’t come from an action in your previous lifetime. It comes from a choice you made here and now.
The Buddha himself argued against the idea that all pain comes from past karma, and in the course of his argument he provided a list of other 14 factors that could give rise to illness. The list comes from the medical beliefs of his day, and although it includes a lot of other causes besides past karma—things like a chemical imbalance in the elements in the body, the change of the seasons, or poor care of the body—all the causes included in the list come under what, in another discourse, he identifies either as past karma or present karma.
So his list conveys two points. First, when the karma leading to a disease plays out, it can often fall under the laws recognized by science. When it does, as when it’s a result of poor diet or body chemistry, the knowledge of medical science can be used to alleviate it—if the person has the good karma to be able to find the proper treatment.
Second, some diseases come primarily from past karma; some primarily from present karma. If it’s a present-karma disease—coming, say, from poor treatment of the body or unhealthy attitudes in the mind—it can go away when the present karma changes. If it’s a past-karma
disease, there are times when treatment in the present can make it go away —in which case, the seeds of good karma are ripening to counteract the effect of bad karma seeds. But there are also times when the past karma is so strong that no treatment will help it. In cases like this, though, your present karma—your attitudes, intentions, and mental skills—can be changed so that even in the face of the illness in the body, your mind doesn’t have to suffer.