In Lincoln's time, this was not known. The Lincoln biography co-authored by his law partner, William Herndon 3b, describes the milk sickness:
In the fall of 1818, the scantily settled region in the vicinity of Pigeon Creek [Indiana] ... suffered a visitation of that dread disease common in the West in early days, and known in the vernacular of the frontier as "the milk sick."In October 1818, Lincoln's mother became ill. He was then nine years old. Herndon continues 3c:
It hovered like a spectre over the Pigeon Creek settlement for over ten years, and its fatal visitation and inroads among the Lincolns, Hankses, and Sparrows finally drove that contingent into Illinois.
To this day the medical profession has never agreed upon any definite cause for the malady, nor have they in all their scientific wrangling determined exactly what the disease itself is. A physician, who has in his practice met a number of cases, describes the symptoms to be "a whitish coat on the tongue, burning sensation of the stomach, severe vomiting, obstinate constipation of the bowels, coolness of the extremities, great restlessness and jactitation, pulse rather small, somewhat more frequent than natural, and lightly chorded. In the course of the disease the coat on the tongue becomes brownish and dark, the countenance dejected, and the prostration of the patient is great. A fatal termination may take place in sixty hours, or life may be prolonged for a period of fourteen days. These are the symptoms of the disease in an acute form. Sometimes it runs into the chronic form, or it may assume that form from the commencement, and after months or years the patient may finally die or recover only a partial degree of health."
Her sufferings, however, were destined to be of brief duration. Within a week she too rested from her labors. ... Abe and his sister Sarah waited on their mother, and did the little jobs and errands required of them. There was no physician nearer than thirty-five miles. The mother knew she was going to die, and called the children to her bedside. She was very weak, and the children leaned over while she gave her last message.Herndon was a tireless researcher, but modern historians believe he sometimes over-dramatized facts.
Amid the miserable surroundings of a home in the wilderness Nancy Hanks passed across the dark river. Though of lowly birth, the victim of poverty and hard usage, she takes a place in history as the mother of a son who liberated a race of men. 3d
A decade later, milk sickness was still present:
The winter of 1829 was marked by abother visitation of that dreaded disease, "the milk sick." It was making the usual ravages among the cattle. Human victims were falling before it every day, and it caused the usual stampede in southern Indiana. Dennis Hanks, discouraged by the prospect and grieving over the loss of his stock, proposed a move further westward. Returning emigrants had brought encouraging news of the newly developed state of Illinois. ... The proposition of Dennis met with the general assent of the Lincoln family. 3e
a pp.106-115, 140-145Comment: More information at: http://www.physical-lincoln.com/
a pp.206-310Comment: More information at: http://www.physical-lincoln.com/
a p.56 b pp.25-26 c pp.26-27 d pp.27 e pp.56-57