What causes back acne and is it completely treatable? also, will scars eventually disappear?

Bacteria, yes: Sebaceous gland ductal obstruction, secondary infection common. Usually low grade. It not understood completely why. Low dose prolonged antibiotic, local care and vit. Supplement will help.

Complex illness: Acne involving the trunk, like all acne, is a complex, non-lifestyle-related, non-hygiene-related, chronic illness that will eventualy remit with age but at a time that cannot be predicted. Systemic medication is usually required when it's on the trunk. The oral contraceptive pill may be a choice for you. The scars will not disappear on their own; i'm simply sorry you were not treated early.

Treatable: Acne is caused by inheritance, plugging of pores and hormones. It is treatable. See a doctor for treatment options.