Passage: At the time, none, not even d'Hérelle, knew exactly what a phage was.  D'Hérelle claimed that it was a biological organism that reproduces, somehow feeding off bacteria. Others, the Nobelist Jules Bordet chief among them, theorized that phages were inanimate chemicals, enzymes specifically, that were already present in bacteria, and only trigger the release of similar proteins, killing the bacteria in the process. Due to this uncertainty, and d'Herelle using phages without much hesitation on humans, his work was under constant attack from many other scientists. It was not until the first phage was observed under an electron microscope by Helmut Ruska in 1939 that its true nature was established.

Question: What was one reason that D'Herelle wasn't taking seriously by his colleagues?
using phages without much hesitation on humans