The reason that the Professor continues to employ Zoidberg as the staff physician, despite his apparent incompetence, is revealed in the sixth-season episode, "The Tip of the Zoidberg". The Professor is under the impression that he had been infected with the fatal disease hyper-malaria, contracted during a covert quasi-military mission. The Professor had asked Zoidberg to promise to kill him when the latent disease manifested itself, and Zoidberg agreed. In that episode it is also explained that Zoidberg is, in fact, a very competent physician "for all other species except humans". Planet Express' main competitor, Mom, whom Zoidberg knows on a first name basis as "Carol", states that Zoidberg is "the best in the business" when it comes to alien anatomy; "at his price level". Unlike all of the other characters, Mom addresses Zoidberg with great respect and admiration. Mom tells Zoidberg that he could have been a millionaire with his own research lab if he had worked for her. Mom also asks Zoidberg why he has stayed with Farnsworth through the years; Zoidberg replied that it is because Farnsworth is his friend.