The sudden appearance of many small red pimples on the face may be caused by acne, folliculitis and other reasons. 1. Acne: It occurs in adolescents, due to elevated androgen levels, excessive sebum secretion, and propagation of Propionibacterium acnes, which may cause acne on the face, and it will be manifested as a lot of small red pimples on the face all of a sudden. 2. Folliculitis: this disease is mainly caused by bacterial infection of the skin, trauma or excessive sweating and other reasons can be triggered, under the stimulation of inflammation, can lead to the affected area of the skin appeared red pimples, pustules, preferably in the head, face, neck and other parts of the body, which may lead to a lot of red pimples suddenly appeared on the face. The sudden appearance of a lot of red small pimples on the face may also be caused by other reasons, you should seek medical advice as soon as possible, after clarifying the causative factors, by the doctor’s guidance for treatment.