Let us remind ourselves that Gregor wakes up one morning to find himself completely transformed from a human into some kind of insect. No remnant of his humanity, in terms of his physical form, remains. Whilst it is impossible to find a disease that will act so quickly, there is one disease that springs to mind that can transform the body very slowly, making it appear as if bits of it are no longer human. This disease is called elephantiasis. I have included some links to it below to give you more information, but here are some basic facts to get you started.
Elephantiasis is a disease whose symptoms are the thickening of the skin, especially in the legs and gential areas. In some cases, infection has resulted in the scrotum swelling up so much that it reaches the size of a basketball. There is a famous film called The Elephant Man which focuses on the life of one person effected by this disease. The way in which this illness slowly transforms the body from a recognisably human form into something completely different would make it a good choice for the assignment you have been given. Remember, however, that the true focus of this short story is not on the disease as such that transforms Gregor, but rather the way that his metamorphosis leads him to be treated by others.