Is celiac disease hereditary? I have it, but nobody else in my family does.

Celiac is hereditary. The celiac disease genes hla-dq2 or hla-dq8 are hereditary and are in almost all celiac patients. The genetics of the remaining small fraction of patients has not been figured out yet. Two parents could each have only one dq2 or dq8 gene, and have very mild symptoms, but their child could have inherited both the dq2 and dq8 from his parents. The child then gets a full-blown case of celiac disease.
Yes, but not always. Celiac is genetic, but there is genetic mutation that causes someone with no history to develop it. Also, there may be a family history in a distant relative of close relative with mild or sub-acute symptoms who was never tested and "suffered silently".