Question

a wet handkerchief is placed on forehead of a person suffering from high fever

Answer 1

During high fever, the internal body temperature of a person is very high. High temperature may effect the brain. Thus, to reduce body temperature, a cold wet handkerchief is placed on the forehead.
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Answer 2

A 'wet handkerchief' is placed on the 'forehead of a person' suffering from 'high fever' to lower down the temperature and bring down fever.  

EXPLANATION:

The water from the handkerchief absorbs the latent heat from the body of the person suffering from fever thereby bringing down the body temperature. The wet handkerchief brings relief to the person suffering from high fever and helps the person feel better by soothing the patient.