“Nothing is more punitive than to give a disease a meaning — that meaning being invariably a moralistic one. Any important disease whose causality is murky, and for which treatment is ineffectual, tends to be awash in significance. First, the subjects of deepest dread (corruption, decay, pollution, anomie, weakness) are identified with the disease. The disease itself becomes a metaphor. Then, in the name of the disease (that is, using it as a metaphor), that horror is imposed on other things. The disease becomes adjectival. Something is said to be disease-like, meaning that it is disgusting or ugly. In French, a moldering stone façade is still lépreuse [leprous].”
This book was published in 1977. The disease I see most often given metaphorical status these days is mental illness.