American pathologist who discovered the causative organisms and mode of transmission of Rocky Mountain spotted fever and epidemic typhus (known in Mexico, where Ricketts worked for a time and died of typhus, as tabardillo).

Ricketts graduated in medicine from Northwestern University, Chicago, and in 1902 joined the faculty of the University of Chicago. In the spring of 1906 he demonstrated that Rocky Mountain spotted fever could be transmitted to a healthy animal by the bite of a certain tick. Two years later he described the causative microorganism; he found it in the blood of the infected animals and also in the ticks and their eggs.

In 1909 Ricketts and his assistant, Russel M. Wilder, went to Mexico City to study epidemic typhus. They found that it was transmitted by the body louse (Pediculus humanus) and located the disease-causing organism both in the blood of the victim and in the bodies of the lice. Before he succumbed to typhus later that year, Ricketts showed that the disease could be transmitted to monkeys, which, after recovering, would develop immunity to the disease. In memory of Ricketts, the genus rickettsia was established for the causative organisms that he had identified.