Dr Ewald Got Interested in Host-Pathogen Interactions as a Result

Dr. Ewald got interested in host-pathogen interactions as a result of considering what question under what circumstance?

A) He came down with malaria while doing a field study and wondered where it came from.
B) He had a severe case of diarrhea and wondered whether that was his body's way of getting rid of an organism, or the organism's way of infecting the next host.
C) His mother developed Alzheimer's disease and he wondered whether it was possible that the disease resulted from a chronic, long-term infection.
D) He was director of a program of disease evolution and he wondered whether heart disease was the result of the evolution of a chronic infection.
E) He actually had several experiences with diseases and in every case wondered how the host evolved in the presence of that particular pathogen.