I've had diarrhea off and on for the last month. what could it be?

Possibilities...: First define the problem (soft or frequent stool?), circumstances under which diarrhea develops (malabsorption, pancreatic maldigestion, food intolerances, stressful times, exposure to ill individuals/infections, food poisoning, ischemic gut, inflammatory colitis, etc.), severity (stool volume, dehydration, blood, night-time awakening, weight loss), ; duration. The "cure" depends on the above.