What illness does Marguerite come down with?

Marguerite suffers from concer of the brain.

IT WAS, THEY would determine, a tumor on the brain, and her decline would be like many others, treatments that made the disease seem mild, hair falling out in patches, mornings spent with noisy radiation machines and evenings spent vomiting in a hospital toilet.

In the final days, when cancer was ruled the victor, the doctors said only, "Rest. Take it easy."