Becoming ill is a part of life, a hazard of life, a consequence of life. There are many causes: exposure to dangerous substances, stress, disappointment, the desire to retreat from the challenge of one’s life, exposure to other people who are ill, depression, grief. Any or all of these things may be present to initiate illness.
Perseverance is a very important ability, but like all important abilities it must have its right application, and it must be understood within a larger scheme of things—a larger context. Take even the most virtuous ability, set it apart from an important purpose, and it turns into an aberrant form of behavior, a problem to be solved, a liability rather than an asset.