Howard Taylor Ricketts was an American pathologist who discovered the class of disease-carrying microorganisms now known as Rickettsia. These microorganisms have characteristics of both viruses and bacteria, and are responsible for lethal human diseases such as typhus and Rocky Mountain spotted fever. Though he died young, he made an important contribution to the study of vaccines and immunity.

Ricketts was born on a farm in Findlay, Ohio, in 1871. He attended Northwestern University and then graduated from the University of Nebraska. After receiving his bachelor's degree, he went back to Northwestern for medical school. He eventually became a pathologist at Chicago's Rush Medical College. He then went to Europe and studied microbiology in Berlin and at the Pasteur Institute. In 1902 he accepted a position as associate professor of pathology at the University of Chicago. There he published research on infection and immunity.

While Ricketts was on vacation in Montana...