The Origin of the word Ghost is curious.

"The first significance of the word, as well as 'spirit,' is breath, or wind." It is of Anglo-Saxon origin, and is from gust, the wind. Hence, a gust of wind. The Irish –word goath, wind, comes nearer to the modern English pronunciation, and shows how easily it could have been corrupted to ghost.

It is easy to imagine the good old Saxon ladies, sitting around the evening fireside, and just as one of them has finished some marvellous story of that superstitious age, they are startled by a sudden blast of wind, sweeping around the gabled cottage, and her listeners exclaim, in suppressed breath, —

" Hark! There's a fearful gust!"

The transit from gust to ghost is easily done. The clothes spread upon the bushes without, or pinned to the lines, flapping in the night air, are seen through the shutterless windows, and they become the object of attraction. The effect supersedes the cause, and the clothes become the gust, goath, or ghost! The clothes, necessarily, must be white, or they could not be seen in the night time!

Hence a ghost is always clothed in white. Therefore the wind (gust) is no longer the ghost, but any white object seen moving in the night air.
The funny side of physic : or, The mysteries of medicine, presenting the humorous and serious sides of medical practice. An exposé of medical humbugs, quacks, and charlatans in all ages and all countries (1874)
A. D.(Addison Darre) Crabtre
http://archive.org/details/funnysideofphysi01unse

QUACKS AND QUACKERY

Quackery and the love of being quacked, are in human nature as weeds are in our fields.
Dr. J. Brown, Spare Hours.

They are Quack-salvers, Fellowes that live by senting oyles and drugs.
Ben Jonson, Volpone, Act n, Scene 2.

These, like quacks in Medicine, excite the malady to profit by the cure, and retard the cure to augment the fees.
Washington Irving.

Here also they have, every night in summer, a world of Montebanks, Ciarlatani, and such stuff, who together with their remedies, strive to please the People with their little Comedies, Fopet-plays and songs.
K. Lassels, Voy. Ital. : 1698.

Le monde n'a Jamais manque de charlatans ; cette science, de taut temps, fut en professeurs tres fertile.
La Fontaine.

He took himself to be no mean Doctour, who being guilty of no Greek, and being demanded why it was called an hectic fever ; 'because,' saith he, ' of an hecking cough, which ever attendeth that disease.'
Thomas Fuller, The Holy State.

Man is a dupable animal. Quacks in Medicine, quacks in Religion, and quacks in Politics know this and act upon that knowledge. There is scarcely anyone who may not, like a trout, be taken by tickling.
Robert Southet.

Quack doctors are indeed pompous, self-sufficient, affectedly solemn, renal and unfeeling with a vengeance.
ViCESiMTJS KNOX, D.D.

If Satan has ever succeeded in compressing a greater amount of concentrated mendacity into one set of human hodies, above every other description, it is in the advertising quacks.
Pacific Medical and Surgical Journal.

The bold and unblushing assertion of the empiric, of a never-failing remedy, constantly reiterated, inspires confidence in the invalid, and not unfrequently tends by its operation on the mind, to assist in the eradication of disorder.
Thos. J. Pettigrew, F.R.S.

The word quack, meaning a charlatan, is an abbreviation of quack-salver. To quack is to utter a harsh, croaking sound, like a duck; and hence secondarily, to talk noisily and to make vain and loud pretensions.' And a salver is one who undertakes to perform cures by the application of ointments or cerates. Hence the term quack-salver was commonly used in the seventeenth century, signifying an ignorant person, who was wont to extol the curative virtues of his salves. Now we see, said Francis Bacon, in " The Advancement of Learning," the weakness and credulity of men. For they will often prefer a mountebank or witch before a learned physician. And therefore the poets were clear-sighted in discerning this extreme folly, when they made Esculapius and Circe brother and sister. For in all times, in the opinion of the multitude, witches, old women and impostors have had a competition with physicians.
Primitive psycho-therapy and quackery (1910)
Robert Means Lawrence, 1847-1935