What does the doctor in chapter 18 reveal to know about Yossarian ?

Yossarian tries to avoid the war by claiming he has abdominal problems. An arrogant English intern informs him that this condition can be easily cured; on the other hand, if Yossarian has a liver condition, he can fool the doctors for weeks, since the liver is a great mystery. Despite Yossarian’s claim that he has liver pain, the other doctors say he is well and must return to the front.