My son was diagnosed with this when he was about 3-4 years old. The Dr. said it was an allergic reaction to the strep virus. He's 10 now and has never had it again.
Ruby:
No one knows what triggers Henoch-Schonlein purpura. It's not contagious. No precautions are useful. It's rare, though: why are you concerned about preventing such a rare disease? If you're worried about inheriting it, it isn't hereditary either.
Best.
Dr. Rockoff