Crohn's disease:

C. Tends to occur in families.

DISCUSSION: The cause of Crohn's disease is unknown. No specific microorganism has been identified as a pathogen, and no clear-cut environmental factor, such as smoking, has been implicated, even though many patients with Crohn's disease are heavy smokers. The disease does tend to occur in families. It is more common among Jews than Asians and among people who live in temperate climates than those in tropical ones.