[4] The American politician Charles Sumner, who visited Louis and observed him teaching at the Hôtel-Dieu de Paris, described him as "a tall man, with a countenance that seems quite passive.

[1][2] He returned to Paris where he worked, initially without pay,[1] at a hospital for seven years, collecting the case histories of thousands of patients and performing hundreds of autopsies.

He eventually wrote studies on the treatment of tuberculosis and typhoid fever, and produced the "numerical method" for evaluating the effectiveness of therapies.

[2] Starting in 1823, Louis began publishing the results of his research on a variety of topics, numerically analyzing information gathered from his case studies and autopsies.

[6] Louis disagreed, publishing a paper in 1828 to that fact[1] (expanded in 1834 to a book-length treatise in the American Journal of the Medical Sciences entitled "An essay on clinical instruction"), which demonstrated that the use of bloodletting for pneumonia was ineffective.

[1] Louis is also credited by some for standardizing the patient history, starting with questions about general health and narrowing down to specific symptoms.

[10] Louis was mentor to Oliver Wendell Holmes Sr. during the younger man's training in Paris and strongly influenced his skeptical outlook.