In the ill state, the legacy system's health has deteriorated to the point that some kind of non-routine intervention is required. In the terminally ill state, the life of the system can be prolonged by extraordinary life support, but heroic measures are required and are often not economically justified. We will elaborate further on each of these states and the implication for possible treatments in each state. When a system is healthy, the enterprise strategy and the legacy system are in harmony. Either the legacy system is satisfactorily handling current enterprise needs or the needs are changing in relatively minor ways such that the legacy system can be updated and maintained in a timely and economical fashion.
Corrective maintenance refers to the correction of errors in the legacy system.
When a system is ill, it may be that way for one of three reasons. Either it was created that way, its internal systems deteriorated over time due to advancing age, or some external environmental influence caused the illness. So it is with legacy systems. Some are poorly designed and constructed to start with. This situation is frequently observed, but it is understandable as we continue to learn better ways to build systems.