The common possible causes of sudden growth of many pimples on the back are acne, urticaria and folliculitis. 1. Acne: frequent consumption of sweet and greasy food leads to increased sebum secretion and blockage of follicle openings, and many pimples may appear on the back. 2. Urticaria: The patient comes into contact with allergens, or ingests food that can easily trigger allergies, resulting in the appearance of many small bumps on the back, accompanied by an obvious itching sensation. 3. Folliculitis: Folliculitis caused by bacterial or fungal infection may lead to the appearance of many pimples on the back. It is recommended to pay attention to personal hygiene, wash in time after sweating, and maintain a healthy diet. If necessary, go to the dermatology department of the hospital to look for the cause and treat it under the guidance of the doctor.