Riyad Us-Saliheen (Gardens of the Righteous)

Chapter 322
Undesirability of Reviling Fever

1726. Jabir (May Allah be pleased with him) reported: The Messenger of Allah (PBUH) visited Umm Sa'ib (or Umm Musaiyyab) and asked her, "What ails you O Umm Sa'ib (or Umm Musaiyyab)? You are shivering.'' She replied: "It is a fever, may Allah not bless it!'' He said to her, "Do not revile fever, for it cleanses out the sins of the sons of Adam in the same way that a furnace removes the dirt of iron.''
[Muslim].

Commentary:  We learn from this Hadith that as diseases are means of expiation of sins, one should not condemn them. More so for the person that they are a part of fate - the Will of Allah. However, there is no harm in taking necessary measures to overcome them because Shari'ah makes it obligatory to do so..