Susan Sontag is using her essay to argue against the prevalent practice of using metaphors to describe illnesses and treatments associated with them. She presents and defends the thesis that health conditions are physical conditions with identifiable causes and courses of treatment based in medical knowledge and expertise. Using the language of metaphors to describe what is happening is not helpful, in her opinion.
The most truthful way of regarding illness--and the healthiest way of being ill--is one most purified of, most resistant to, metaphoric thinking.
She particularly examines the ways in which tuberculosis was perceived and treated in the past and in which cancer is thought of in the present. Tubercuclosis was thought to be an illness that particularly afflicted persons with artistic tendencies, those who were passionate about life and who wasted away because of the intensity with which they lived life. Cancer is described as "invading" organs; the "battle against cancer" is "fought" with all the "weapons" medical science can provide.
Her main point is that such thinking is not helpful for the patient. She supports a straightforward and strictly factual analysis of the physical causes and effects of the illness and of the actions available to treat it.