"Watch yourselves very carefully...." [Va'Etchanan 4:15]
So much of physical health depends on spiritual health. If in olden days emphasis was placed on "mens sana in corpore sano" [a sound mind in a healthy body], in our days it is a matter of general conviction that even a small defect spiritually, causes a grievous defect physically; and the healthier the spirit and the greater its preponderance over the physical body - the greater its ability to correct or overcome physical shortcomings; so much so, that in many cases even physical treatments, prescriptions and drugs are considerably more effective if they are accompanied by the patient's strong will and determination to cooperate.
Note that Rambam stresses how "having a totally healthy body is among the paths of (serving) God", a point emphasized further by the Mezritcher Maggid.
Since physical health depends on spiritual health, if a person becomes ill, G-d forbid, he should search his past deeds to try to identify what shortcoming may have caused the illness. However, this approach should be taken only regarding one's own lack of physical health. When one sees that another person is sick, one should not think that this was caused by a spiritual shortcoming, since we are told "Do not judge your fellow until you have stood in his place" [Avos 2:4, see Tanya Ch 30]. One's first reaction to a sick person should be, to the contrary, that his sickness may well have been caused by spiritual health, as he may have weakened his body through fasting, in the process of doing teshuvah [see the Alter Rebbe's Shulchan Aruch].
The statement of the Zohar that "the weakness of the body is the strength of the soul" does not mean to say that a weakening of the body itself brings about spiritual growth. Rather, the intent of the Zohar is that the desire for physicality, for its own sake, is counter-productive to a person's spiritual growth.
Source: Likutei Sichos of the Lubavitcher Rebbe
As a youth, R' Yechezkel Abramsky was sent to Siberia. The cold was unbearable, and temperatures dropped to as low as 40 degrees below zero.
Wearing only light clothing, young Yechezkel stood in line along with the other Jews who had been exiled to that forsaken part of the world. They were all trembling from the cold.
"Listen Jews!" shouted the commanding officer. "Every morning you are to remove your shoes and run barefoot in the snow for the duration of an hour. Anyone who dares violate this order will be severly punished!"
R' Abramsky, who was a weak and frail youth, was frightened by this cruel order. Back at his warm home, his loving mother had always tended to him, dressing him in warm clothing and scarves, but now he would have to run barefoot in the snow!
He lifted his eyes to Heaven and pleaded with Hashem: "Master of the World" he said, "you have exhorted us in Your holy Torah to "watch yourselves very carefully". In truth, man is usually able to take care of his health by wearing warm clothing, but here in this Siberian labor camp, we are unable to do so. We therefore cannot be held responsible for our health. I therefore beg of You, Master of the World, watch over and us and protect us!"
Amazingly, throughout his entire stay in Siberia, R' Abramsky did not get sick even once.