After his schooling, Coley began working at New York Hospital, now Weill Cornell Medical Center, as a surgical intern.

Despite receiving such a critical surgery as forearm amputation, Bessie died just ten weeks later due to widespread metastasis of the original cancer.

To the astonishment of the doctors in the hospital at the time, Stein's round cell sarcoma seemingly vanished after being diagnosed with erysipelas, a skin infection now known to be caused by the bacterium Streptococcus pyogenes.

[5] Curious to discover the reason for the man's remission, Coley decided to search for the hospital's discharged patient, whom he eventually found in Manhattan with no trace of cancer left in his body.

The formula change to the use of dead bacteria drastically reduced the risk of death in patients; however, Coley's toxins still remained controversial in the medical community.

Although Coley claimed successful treatment of hundreds of patients, the absence of proven benefit or reproducibility led to broader emphasis on surgery and on the newly developing field of radiation therapy.

However, after several years of experience, Coley came to the conclusion that the effect of that primitive x-ray therapy in the untrained hands of experimenters was localized, temporary, and not curative.

His contemporary critics cited the dangerous and unpredictable effects, predominantly the fever caused by the bacteria, that the vaccine had upon individuals weakened by cancer.

According to the analyses of Coley Nauts and Starnes, treatment success correlated with length of therapy and the fevers induced by the toxins.

He was survived by his wife and two children who continuously worked after his death to preserve Coley's legacy in the field of cancer research.