As is the case with regard to many etymological roots in Hebrew (and its sister language, Aramaic), the two-letter root of "sickness" (chal) possesses variant meanings, amongst which are apparent opposites.
First, the root for "sick" can mean either "weak" or "strong." In the story of Samson and Delilah–when Samson revealed to Delilah the secret of his strength–"to become sick" means "to become weak":
He told her all his heart, and said to her:
"A razor has never come upon my head,
For I have been a nazarite unto God from my mother’s womb.
If I be shaven, then my strength will go from me,
And I shall become weak [vechaliti],
And be like any other man."
In Chassidut, we learn from this verse–"and I shall become weak and be like any other man"–that the very concept of "sickness" is relative in nature. What for any other man might be a state of good health and well-being, for Samson is a state of illness. For Samson, to lose his strength is to lose his very "sex" or male status; to become as a woman, one of the weak sex (what we consider to be the natural, healthy, menstrual cycle of the woman is regarded in Torah as an innate state of "illness," one of the curses to Eve resulting from her sin of eating the fruit of the Tree of Knowledge of good and evil).
In contrast, the word chayil, cognate to "sick," means "strength," as stated with regard to the righteous in this world and the World to Come:
They go from strength to strength [ michayil el chayil],
Every one of them shall appear before God in Zion.
A soldier is a chayal. A "woman of valor" is an eshet chayil.
Another pair of opposites, from the root chal (primarily in its Aramaic usage), is chal in the sense of "bitter" or "tart" and chal in the sense of "sweet."
The relation between bitterness and disease is apparent from the fact that the name of the physical limb, which according to the sages is the seat of all disease, means "bitter." It is the gall-bladder–in Hebrew marah, from mar, "bitter"–of which is said that all of the eighty-three illnesses (the numerical value of the word for "disease" in Hebrew [machalah], as will be explained) that afflict mankind are dependent on the gall-bladder.
The bile of the gall-bladder was termed by the ancients as the "yellow" or "green" humor in the body. In Kabbalah and Chassidut, it is associated in particular with the body’s natural desire to pursue physical pleasure. It is further referred to as "the lower waters" of creation, in contrast to "the higher waters," spiritual and Divine pleasure. When the two waters are separated and distanced from one another, illness ensues.
Even when imprisoned, caught up in the clutches of physical pleasure, estranged from its spiritual source, the inner consciousness of the lower waters still cries out to God in existential bitterness: "we also desire to be in the presence of God, to experience Divine pleasure as do the higher waters." Thus, we are taught in Chassidut that disease and its remedy depend upon the rectification or redirection of the "pleasure principle" of the soul, the transformation of the bitter (the apparent sweetness of profane physical pleasures) to the (truly) sweet (pleasure in the Divine, the experience of the ultimate Divine unity underlying all reality, both physical and spiritual).
Based upon these two phenomena–the etymological relationship of "weakness" to "strength" and "bitterness" to "sweetness," all in conjunction with the word for "sickness"–we are taught in Chassidut that "sickness" is actually an existential intermediate state of being–the intermediate between life and death. "Life" is a state of holiness (that which is truly holy lives forever), whereas "death" is a state of profanity (the origin of all impurity). The intermediate state is the realm of the "mundane" (chulin from the root chal, "sickness"). In Kabbalah it is referred to as kelipat nogah, "the translucent shell," the intermediate between transparency (clear revelation of the Divine nature of reality) and opacity (hiddenness—non-recognition—of the Divine).
Thus, sickness may serve as a bridge in two directions: from life to death or from death to life. To recover from sickness is to be reborn, alive once more. One actually became sick in order to return stronger and healthier than ever before. Sometimes, as in the case of Mashiach, one becomes sick in order to connect and thereby elevate fallen souls. Mashiach suffers, in actual physical disease, in order to redeem–to raise from the realm of the dead–the people of Israel and all of mankind. In the ever resounding words of Isaiah:
…he [Mashiach] is a man of pain,
acquainted with illness [choli, ] …
Verily, he has borne our illness [choloyeinu, ],
And he has suffered our pain.
As taught in Chassidut, each of us possesses a spark of Mashiach. An essential part of the Divine providence at play in one becoming sick is that he come to identify himself with all suffering souls, and, in supplication to God, intend to recover and be redeemed together with them all.