Historically, tropical disease research was almost wholly concerned with parasitic diseases. Parasitic diseases are those caused by protozoa and helminths. The schools of tropical medicine in the U.S. and Europe focused largely on those organisms. The interesting thing is that when we began to look more carefully at mortality figures we found out that the greatest killers in the developing world were not the parasitic diseases, but the diarrheal diseases and acute respiratory infections caused largely by bacteria and viruses.