Hot baths are not recommended for adults with fever.
Fever is a dysfunction of the body’s thermoregulatory center due to various reasons. Under normal circumstances the body’s heat production and heat dissipation remain relatively balanced, while in fever, skin blood vessels and vertical hair muscles contract, sweating decreases, the body’s heat production increases and heat dissipation decreases, resulting in the body temperature rising or maintained at a higher state.
When taking a hot bath, the temperature of the water is higher, resulting in the body needing to dissipate heat through evaporation. However, usually there is no way to sweat when you have a fever, and sometimes there is no sweating at all, so you cannot dissipate heat through sweating, which results in the possibility of an even higher body temperature, and so taking a hot bath when you have a fever is not recommended.
In the course of treatment and management of illness, one should listen to or refer to the advice of a doctor and not dispose of the illness blindly, so as not to cause adverse consequences by improper treatment.