There are some illnesses and diseases that will still occur to a pet that was even taken great care of. Some are just acute and after treatment the pet recovers. However, some are more serious than that and become chronic.
When a pet has a chronic illness then the next stage of care is managing and providing supportive care so that their quality of life is as good as possible. Unfortunately, sometimes making them comfortable and as symptom free as possible is all that can be done for them.