When I was 10, while touching my toes during a routine school medical examination, I was told that my spine had a slight curve. It was, I then thought, the stuff of ghoulish fairy tales. What naughty deed had led to such unnatural punishment? I had come across a word for people with crooked spines in my books: hunchbacks.

It is a word that has re-emerged this week, after Fergie, Duchess of York, said her 12-year-old daughter Eugenie would have become "hunchbacked" if she hadn't had an operation last October to correct a curvature of the spine.

There are two forms of this body-altering condition: kyphosis, characterised by a central hump, personified on celluloid by Victor Hugo's bellringer Quasimodo; and scoliosis (Eugenie's form), when the spine is twisted into an S or C shape, threatening to form a side hump with one shoulder thrust up and forward. Although it commonly occurs during the growth spurts of adolescence, scoliosis can emerge at any time through to old age, when the spinal column can degenerate due to osteoporosis. It is quite common; it has been estimated that as many as three in every 100 people have curvature of the spine in some form. Girls make up the majority of the most serious cases.

My own curve is very slight and insignificant; it has no side effects, and only a trained eye could spot it. But just the mention of a kink in the spine is enough to conjure up demons. Shakespeare's Richard III, portrayed as a "lump of foul deformity", is credited as the first person to be labelled hunchback. His sad physical state, which drove him to extreme wickedness, was said to be the result of witchcraft. Classic children's author Frances Hodgson Burnett, best known for The Secret Garden, wrote The Little Hunchback Zia in which a poor beggar boy is cured of his curvature by faith, as the devils are driven out of him.

Fergie seems to have read the same stories I did as a child. In her account of her daughter's condition, medical and moral wellbeing are intimately connected. She said Eugenie's "bones had oscillated to such a degree that by the time she reached 18 she would have been a hunchback. It was horrendous." Only surgery saved her: "She is straight and she will stay straight," said Fergie, as if, consciously or not, referring to the body and soul. No longer threatened with being condemned as a hunchback, Eugenie can stand tall.

It is curious how some medical conditions carry moral weight while others do not. Even diseases for which the sufferer is clearly partly culpable, such as most lung cancers, don't attract nearly as much stigma as a curve in the spine. Yet scoliosis can't be attributed to anything the sufferer does. In 80% of cases, there is no known cause.

So why are lung cancer sufferers called "patients" and pitied, while those with scoliosis are labelled "hunchbacks" and shunned? Is it because scoliosis is physically deforming? So pivotal is an upright spine to our image of a whole human being that Quasimodo has come to signify not just a specific condition, but ugliness in all and any form.

Faced with a future full of hostile stares, any of us would opt for surgery. Yet, although scoliosis is regularly described as "crippling", only in severe cases may there be medical reasons for correction. Surgery can be cosmetic. The reason sufferers undergo it is, quite understandably, to make themselves look and, therefore, feel better. It is to cure, not a disease, but what the medical books themselves call a "deformity".

But is it so inconceivable that some day, somewhere, being a hunchback may be considered desirable? Of course, we all want to be regular, and we all hope our children will be. But cures can only correct the physical condition. Divorce judgments from deformities, and perhaps they appear less devastating. Many people may have a slightly crooked spine. But they may still be upright citizens.