In 1901, he supervised a large project in which thousands of rats were tested for bubonic plague, which many of them were found to be carrying and which it was feared they would spread through the city.
[3] He eventually suffered a physical breakdown and took a leave of absence to recover in Japan, being succeeded in his work by physicians Paul C. Freer and Richard P.
[4] After his recuperation, he returned to the United States to undertake postgraduate education at Johns Hopkins University, before traveling to Berlin to study at the Robert Koch Institute.
He then joined the staff of the Rockefeller Institute for Medical Research, where he remained from 1906-1909 and where he began working with Simon Flexner.
[5] He left the Rockefeller Institute to become a staff pathologist at Chicago's Michael Reese Hospital, where he remained until 1913.