In Chapter 17, Yossarian describes the diseases that may take him or any of the soldiers, just after he enters the hospital:
There were billions of conscientious body cells oxidating away day and night like dumb animals at their complicated job of keeping him alive and healthy, and every one was a potential traitor and foe. There were so many diseases that it took a truly diseased mind to even think about them as often as he and Hungry Joe did.
Yossarian's understanding of the human body contains layers of personification. First Yossarian describes the human body as nothing but billions of cells, an un-personification, as the body is imagined not as a person but as a pile of organic matter. But those cells are "conscientious," doing work every day, "oxidating" all the time. So the cells are re-personified, given human traits, as part of the human body.
But then immediately the cells are compared to "dumb animals," so they are again un-personified. (Yossarian, and Heller, make a swift and disparaging comment regarding people who do conscientious jobs in which they do the same thing every day.) Then, those "dumb animals" are re-personified again, when Yossarian remembers that each one of them could be a "traitor or foe." Being a "traitor or foe" are human traits. So in sum, the human body is imagined as a pile of cells, which are compared to humans, which are compared to animals, which are compared to humans. In all, Yossarian is correct that it takes a "truly diseased mind" to consider the body this way, and more diseased still to be constantly vigilant for any traitor or foe within it.