Rashes on the body may be caused by diseases such as atopic dermatitis, solar dermatitis, and stage 2 syphilis rash. 1. Allergic dermatitis: due to contact with allergy-causing substances or consumption of allergy-causing foods, the body develops a hypersensitivity reaction, resulting in erythema and pimples all over the body, accompanied by itching. 2. Solar dermatitis: the skin is exposed to ultraviolet rays for a long time and an inflammatory reaction occurs, which is manifested as small rice-like pimples, mostly accompanied by burning, pain and itching. 2. Stage II syphilis: If there is a history of sexual contact with patients with syphilis, there is a possibility of syphilis virus, in the second stage can be manifested in the whole body with copper-red patches, can be accompanied by collar-like dandruff, no discomfort. There are many reasons for the rash on the body, the treatment is also different, it is recommended to consult a doctor in a timely manner, to clarify the cause of the disease, and then treat the symptoms.