Heat or high humidity can make many people with multiple sclerosis (MS) experience a temporary worsening of their symptoms. Doctors believe that this occurs because heat causes nerves (whose myelin covering has been destroyed from MS) to conduct electrical signals even less efficiently. Most people with MS avoid hot baths, hot showers, and heated swimming pools. For reasons that are not well understood, extremely cold temperatures and changes in temperature can also cause MS symptoms, usually spasticity, to flare.