Final answer:
Feline infectious peritonitis (FIP) is the disease caused by the mutated form of a harmless intestinal coronavirus.
Step-by-step explanation:
The disease caused by the mutated form of a harmless intestinal coronavirus is Feline infectious peritonitis (FIP). FIP is a viral disease that affects cats and can be fatal. The mutated form of the coronavirus causes the immune system to overreact, leading to inflammation and damage to various organs in the body.