Most every kind of disease causes the body weakness and fatigue at some point. In the beginning of a sickness the body instinctively desires rest in order to better consolidate its resources against the viral or bacterial attack. Frequently, over time, weakness and fatigue reappear with a gradual vengeance if a disease is winning, as the body slowly begins to buckle, having consumed its strength fighting.
Diseases that are especially known for causing weakness and fatigue:
CFS - Chronic Fatigue Syndrome