Sudden acne can be caused by acne, folliculitis, insect bite dermatitis and other reasons. 1. Acne: androgen secretion in the body is exuberant, follicular duct hyperkeratosis, Propionibacterium acnes reproduction, which will cause the follicle secretion increase, can not be discharged in time plus the infection of the formation of acne, resulting in sudden growth of acne. 2. Folliculitis: Inflammatory reaction caused by bacterial or fungal infection of hair follicles will cause congestion, swelling, pain and itching of hair follicles, which will also lead to sudden acne. 3. Insect bite dermatitis: Inflammatory reaction caused by mosquito bites on the skin can also lead to sudden acne. Sudden acne may have other reasons, you need to go to the hospital to check the cause of the disease, so as not to miss the condition of the symptomatic treatment.