Any one of several, unrelated ailments used as a plot device
in Victorian novels. Let's take them in turn:
Congenital VND This affects small to prepubertal children, and is brought about by VND in the mother, which is sometimes in turn caused by malfeasance, which can range from immorality to a stay in India. The child is small, stays sickly, and is usually of more than ordinary innocence and goodness, which, in turn, is signalled by the presence of recessive genes: blonde hair, blue eyes, and a slight build. Such children usually die on the cusp of puberty (most often on Christmas or some other holiday) after giving their favorite toys to the dustman's son. In the later Victorian era, and into the 20th century, such children are saved from death by "country living" and become healthy, well-adjusted adults.
Pubertal VND This affects mostly women at puberty, and is related to greensickness -- iron-deficient anemia surrounding menarche. Symptoms include langor, laziness, light-headedness, and a tendency to faint readily -- otherwise known as "the vapors". (No lie -- this one is for real...) Treated by maturity, eating beef, pelvic massage and sometimes, travel abroad. Clears up immediately upon marriage. (Hmm....)
Adult VND. This is of two kinds: moral, which is occasioned by being too good to live, and immoral, which is contracted through too much sex. The moral variety causes the patient to have much the symptoms of TB, without the gruesome ones, like spitting up blood, plus a few more, such as increased beauty, aesthetic and artistic sense, higher intelligence, and a mindset close to that of being mildly stoned. (Overuse of patent medicine also can account for this.) It is nearly always fatal, but amusing while it lasts. (Opera disease is close to this, but since it's Continental, and usually occurs among the poor, I leave this to a different WU.) Immoral VND is caused, as I said, by too much sex, and has very little to do with VD as such -- it's the sheer will-sapping nature of the act itself that does it. (In one instance, a fatal illness was contracted through one bout.) Symptoms are similar to the moral variety, except with less coughing, a marked darkening of skin and hair, and a sinister craving for whatever laid you up in the first place, along with increased craving for alcohol, opiates, and the like. Fatal, but not until one loses one's job, marriage, and social standing, and is reduced to a squalid rented room.
Any or all of these are excerbated by youth and the financial status of the sufferer: the wealthier the family, and the younger the patient, the more likely they are to have one of these ailments. Beware the false invalid, scourge of English "cozy" mysteries: they may have been bedridden for more than thirty years or more, but are more than able to throw off their lap robes, run down three flights of stairs, and knock off Mr. Boddy before anyone's the wiser...