জ্বর হচ্ছে শারীরিক অসুস্থতার অন্যতম প্রধান লক্ষ্মণ, যা শরীরের স্বাভাবিক তাপমাত্রার সীমার ৩৬–৩৭.২ °সে (৯৬.৮–৯৯.০ °ফা) অধিক তাপমাত্রা নির্দেশ করে।

Fever or pyrexia in humans is a body temperature above the normal range due to an increase in the body's temperature set point in the hypothalamus. There is no single agreed-upon upper limit for normal temperature: sources use values ranging between 37.2 and 38.3 °C in humans. The increase in set point triggers increased muscle contractions and causes a feeling of cold or chills. This results in greater heat production and efforts to conserve heat. When the set point temperature returns to normal, a person feels hot, becomes flushed, and may begin to sweat. Rarely a fever may trigger a febrile seizure, with this being more common in young children. Fevers do not typically go higher than 41 to 42 °C.