Which is often more predictive of the client's success than the characteristics of his or her illness?

Living environment.

Explanation:

The illness may be defined as any kind of disease or sickness that affects the normal body condition. Bacteria, virus and other disease causing organisms can cause illness.

Both the external and internal environmental factors might responsible for the illness. The positive and healthy living environment is important to keep the individual's healthy and free from any illness. The internal living environment represents the physiological condition of the body and the external living environment represents the physical condition like proper sanitation, proper hygiene is important for the client success.

Thus, the answer is living environment.