Jack Osbourne -- I Have Multiple Sclerosis

Jack Osbourne has been diagnosed with multiple sclerosis, the former "Osbournes" star has revealed.

Osbourne tells PEOPLE he learned the news last month ... just two weeks after the birth of his daughter, Pearl Clementine.

Jack tells the mag, "I was just angry and frustrated and kept thinking, 'Why now?' I've got a family and that's what's supposed to be the most important thing."