Babies get red rashes when they are hot, mostly due to urticaria, eczema, etc. 1. Urticaria: a temporary inflammatory congestive disease occurring in the blood vessels of the skin and mucous membranes due to various factors, which may be manifested as small pimples on the skin, or large air masses, wandering, accompanied by obvious itching. 2. Eczema: It occurs on the head and face of infants. It is a kind of allergic skin disease, often manifested as red papules or erythema, scales, skin thickening on the head and face. Symptoms are more pronounced when the temperature rises and sweating is high. Infants get red rashes when they are hot, it may also be caused by other reasons, it is recommended to take the infant to the hospital in time to identify the cause and cooperate with the doctor for treatment.