While working for the Rockefeller Institute, in 1931 Rhoads was invited by hematologist William B. Castle to join his Rockefeller Anemia Commission, to conduct clinical research at Presbyterian Hospital in San Juan, Puerto Rico. This was part of the Rockefeller Foundation's sanitary commission on the island through the International Health Division. Castle's research interest was pernicious iron deficiency anemia, specifically as caused by the parasitic hookworm, which was endemic on the island at rates of 80%, and tropical sprue. An effective treatment for the latter had just been developed, although the disease's causes remained obscure. As recently as 2010, medical journal "Nieto Editores" reported that these conditions continued to cause high mortality in Puerto Ricans. The cause of tropical sprue has still not been identified, but since the 1940s, it can be treated with folic acid and a 3 to 6-month course of antibiotics.