Osbourne, 26, learned the news just two weeks after welcoming his daughter, Pearl Clementine

Jack Osbourne has been diagnosed with multiple sclerosis, he tells PEOPLE exclusively.

Just two weeks after he and fiancée Lisa Stelly welcomed daughter Pearl Clementine, Osbourne, 26, was dealt the shocking news. In an exclusive, sit-down interview and photo shoot with PEOPLE magazine, Jack and his mother Sharon open up about their emotional ordeal.

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“I was just angry and frustrated and kept thinking, ‘Why now?’ ” Jack tells PEOPLE. “I’ve got a family and that’s what’s supposed to be the most important thing.”

Multiple sclerosis is a disease that affects the central nervous system – the brain and spinal cord – and can cause problems with muscle control and strength, vision, balance, feeling, and thinking.

MS differs in each case, with some people going through life with only minor problems and others becoming seriously disabled. Most people fall somewhere between those two extremes.

Visit Health.com for more information about multiple sclerosis.