Erythema on the back of the neck is not necessarily syphilis, but can also be related to allergies, infections, mosquito bites, etc. And syphilis usually grows red spots not limited to the neck. When syphilis develops into stage 2 syphilis, syphilis rash can appear on the skin, manifested as erythema, papules, plaques, etc. Most of them are generalized, and appear in many parts of the body, seldom confined to the neck, if suspected of syphilis, it is necessary to check the serology of syphilis to make judgment. Erythema on the back of the neck may be related to the intake of certain foods, contact with certain allergens, etc. Erythema may also be caused when the neck is infected by pathogens such as Staphylococcus aureus and Candida albicans, etc. Mosquito bites may also lead to the appearance of erythema. Therefore, there are many reasons for erythema on the back of the neck, and it is recommended to consult a doctor in time for a clear diagnosis and symptomatic treatment so as not to delay the condition.