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.
Likutei Sichos of the Lubavitcher Rebbe
ONE SHOULD RECITE TEHILLIM/Psalms 41 FOR SOMEONE THAT IS SICK. Preferrably in Lashon Kodesh/Holy Tongue. However, G-d hears our prayers in any language. Please take the time to listen to a shiur/lesson on chapter 41. It is of utmost importance as this Rabbi explains each verse. There are some that visit the sick/injuried and say...."oh, how horrible - I feel bad for you. I pray you get well soon." And in his heart and on their lips they will whisper to others for the sick/injuried's downfall.
Do take time to feed your soul and listen to this lesson that is for everyone. I have also provided a link that has the loshon kodesh and english side by side.
Here is the audio and here is Tehillim/Psalm 41.