1. He has a severe illness.
2. He has a serious illness.
3. He is severe.
4. He is serious.
(Are both the same and interchangable?)
In sentences 1 and 2, they are interchangeable, but not in sentences 3 and 4.
In sentence 3, "severe" means he treats other people harshly.
In sentence 4, "serious" means he is not joking around!