What is the reason for feeling fear, headaches, tremors, a decrease in faith, and the whispers of Satan after experiencing a feverish sensation in the body? Is it due to sins or the evil eye?

Weak faith and distancing oneself from Allah are among the most dangerous effects of sins and transgressions. It is obligatory for whoever feels hardness of heart or weak faith to repent immediately and increase in effacing good deeds. For when a servant turns to his Lord, Allah draws him near and replaces his hardness with softness, and his distance with nearness, as stated in the Hadith Qudsi: "My servant continues to draw near to Me with supererogatory deeds until I love him...", and "If My servant draws near to Me by a handspan, I draw near to him by an arm's length...". As for whispers (waswas), their cure is by repelling and ignoring them. If a servant is afflicted with a physical illness, he must seek treatment with permissible medicines and go to trustworthy doctors, and use the ruqyah (incantations) established in the Qur'an and Sunnah. Envy (hasad) is real, but not every misfortune should be attributed to it; rather, matters should be placed within their Shar'i (Islamic legal) framework.