Most commonly it is due to sweating and is common in persons who frequent the gym and are unable to shower after working out

Most infections on the back are not acne. Self diagnosis is dangerous. Acne is caused by bacteria. Spend good money getting proper diagnosis from a dermatologist. It will save you years of struggle and side effects from bad medication; it will be cheaper in the long run.

Bacteria causes it. After work outs, sweating can increase "back-ne". A topical antibacterial wash once a day may help, but if a prolonged state of back acne persists an oral antibacterial or topical antibacterial may be needed. Please visit your dermatologist to get more recommendations and for your yearly skin exams.