Dr. Rush did not consider it odd that the entire race should have contracted leprosy:
[T]he influence of unwholsome diet ... combined with greater heat, more savage manners, and bilious fevers, probably produced this disease in the skin among the natives of Africa.
He also dismissed any objections as to its hereditary properties:
[T]he leprosy is the most durable in its descent to posterity, and the most indestructable in its nature of any disease we are acquainted with. In Iceland Dr. Van Troil tells us, it often disappears in the second and third, and appears in the fourth generation.
He went on to list a number of reasons why he believed that their color was a result of leprosy:
1. The leprosy is accompanied in some instances with a black color of the skin.
And citing someone else, he explained that this disease is responsible for the supposed odor of blacks:
"They [lepers] exhale perpetually a peculiar and disagreeable smell, which I can compare to nothing but the smell of a mortified limb." This smell mentioned by Dr. Theiry continues with a small modification in the native African to this day.
4. The leprosy induces a morbid insensibility in the nerves. ... This insensibility belongs in a peculiar manner to the negroes.
5. Lepers are remarkable for having strong venereal desires. This is universal among the negroes... .
6. The big lip, and flat nose so universal among the negroes, are symptoms of the leprosy. I have more than once seen them in the Pennsylvania hospital.
7. The woolly heads of the negroes cannot be accounted for from climate, diet, state of society, or bilious diseases, for all those circumstances, when combined have not produced it in the natives of Asia and America who inhabit similar latitudes. ... [I]t would seem that the leprosy had found its way to the covering of the head, and from the variety of its effects upon the skin, I see no difficulty in admitting that it may ... have produced wool upon the head of the negro... .
But if being black is a disease, why don't people catch it?
I would reply to such objection by remarking in the first place, that the leprosy has in a great degree ceased to be infectious, more especially from contact, and secondly that there are instances in which something like an infectious quality has appeared in the skin of a negro. A white woman in North Carolina not only acquired a dark color, but several of the features of a negro, by marrying and living with a black husband. A similar instance of a change in the color and features of a woman in Buck's county in Pennsylvania has been observed and from a similar cause.
And of course, there's no reason to expect a race of lepers to be unhealthy or short-lived.
It is no objection to the theory I have attempted to establish, that the negroes are as healthy, and long lived as the white people.
So, what did all this mean? Rush, an abolitionist, warned his audience that if blacks were all victims of disease, this warranted further charity on the part of the white man. Further, he called for the discovery of a cure, and toward that goal retold anecdotes of black men becoming white through "bleeding" (a favorite technique of Rush's), "the influence of fear", "oxygenated muriatic acid", and "unripe peaches."
Of course, if anyone suggested that blacks might simply want to remain the way they are, Rush had words for them, too: "[H]owever well they appear to be satisfied with their color, there are many proofs of their preferring that of the white people"--none of which he provided.