Part 1: Identify the disease and parts of the body it effects.
Part 2: Normal function of those areas. Identify the anatomical location and physiological features. In other words, under normal healthy conditions where is it and what does it do for the body? Pay special attention to ways in which it maintains homeostasis.
Part 3: Explain how the disease you are writing about disrupts the physiological process and anatomical structure (the anatomical structure may not change so this point may not be relevant). How does the body adapt to try and offset the negative effects as much as possible? How does it later develop into a chronic condition?
Part 4: Concluding remarks