Ab Hurayrah Allah be pleased with him said:

There is no illness that afflicts me more beloved to me than fever: it enters every part of me and [because of it] Allah the Mighty and Sublime gives every part of me its share of reward.

Al-Bukhr, Al-Adab Al-Mufrad. See Shaykh Al-Albns Sahh Al-Adab Al-Mufrad Vol.1 p198.