What causes body acne and how can i prevent it?

Not preventable: It's manageable. You'll need both a topical agent such as benozyl peroxide or retinoic acid, and prescription-strength meds, probably oral, such as oral tetracycline, spironolactone, or the oral contraceptive pill. Acne is a major liability unrelated to lifestyle, diet, or anything else under your control. As an adult, you'll need to work with your physician to clear it up. Best wishes