Death is a fascinating thing. The human mind continually returns and returns to death, to mortality, to immortality, damnation, salvation. Some fear death, some seek it but it is in our nature to wonder at the limits of human life, at least. When you are sick like this you begin to wonder too much. Death is at your shoulder, death is your shadow, your scent, your waking and dreaming companion. You cannot help, when sleep begins to touch your eyes, but think: What if? What if? And in that question, there is a longing, too much like the longing of a young girl in love.
You will get sick again because sick is what you know.
— Marya Hornbacher, Wasted
Toska - noun /ˈtō-skə/ - Russian word roughly translated as sadness, melancholia, lugubriousness.
“No single word in English renders all the shades of toska. At its deepest and most painful, it is a sensation of great spiritual anguish, often without any specific cause. At less morbid levels it is a dull ache of the soul, a longing with nothing to long for, a sick pining, a vague restlessness, mental throes, yearning. In particular cases it may be the desire for somebody of something specific, nostalgia, love-sickness. At the lowest level it grades into ennui, boredom.
— ― Vladimir Nabokov