All the hair follicles a person will ever have are present at birth. Follicles that die are not replaced and never again produce a hair. It follows that any condition that destroys large number of follicles makes the hair permanently thin. This is what happens in male baldness and in certain rare, scarring diseases of the scalp. Proprietary hair-restorers will do nothing whatsoever to help these conditions.
Nothing can make a dead hair follicle produce new roots.
Illness, prolonged drug therapy or emotional stress can also affect hair growth and its quality, so that patches of thinning hair appear. However, this condition, called alopecia, is usually only temporary. Quite often the result is simply to shorten the hair's life cycle so that it enters its resting phase prematurely. Rather than cause hair loss, illness may make the hair thinner in diameter, more liable to split and less shiny. If a serious illness sends the majority of hair into a simultaneous resting phase, their loss will show up as a thinning of hair or even as bald patches but within a matter of months the new hair should grow through.