Do not curse the fever, for it removes the sins of the children of Adam the same way the bellows remove the impurities of iron

Jābir (may Allah be pleased with him) informs us in this Hadīth that the Prophet (may Allah's peace and blessings be upon him) entered the house of Umm As-Sā'ib and found her shivering. So he asked her why she was shivering and she said that it was the fever. The fever, or "humma" is a rise in the body temperature. It is an illness of many types.
"May Allah not bless it": Umm As-Sā'ib supplicated against the fever that afflicted her, so the Prophet (may Allah's peace and blessings be upon him) said: "Do not curse the fever." He forbade her from cursing the fever because it is from Allah's actions. Everything that is from Allah's actions, it is not permissible to curse it, since cursing the action implies cursing its Creator, the Most High. It is for this reason that the Prophet (may Allah's peace and blessings be upon him) said in another Hadīth: "Do not curse the time, for, indeed, Allah is the time."
It is the duty of the person who is afflicted with fever or the like of it to be patient and seek reward from Allah.
"For it removes the sins of the children of Adam the same way the bellows remove the impurities of iron", i.e. fever is a reason for the expiation of sins and the raising of status. Just as melting iron over fire removes its impurities and alloys, fever removes one's minor sins and purifies him of them.