How does David McCullough use imagery in The Path Between the Seas?

Imagery:

"And all the while, in the lovely gardens surrounding the hospital, thousands of ring-shaped pottery dishes filled with water to protect plants and flowers from ants provided perfect breeding places for mosquitoes. Even in the sick wards themselves the legs of the beds were placed in shallow basins of water, again to keep the ants away, and there were no screens in any of the windows or doors. Patients, furthermore, were placed in the wards according to nationality, rather than by disease, with the result that every ward had its malaria and yellow-fever cases. As Dr. Gorgas was to write, had the French been consciously trying to propagate malaria and yellow fever, they could not have provided conditions better suited for the purpose."