Why are parasitic diseases difficult to control?

Agree: Parasitic infections occur more commonly in 3rd world countries where financial resources and conditions are less than optimal in dealing with these conditions.

Hygiene, sanitation: Actually, most parastic diseases are easy to control. That's why the most important ones are now very rare in the US and most industrialized countries, i.e. no more hookworm or trichinosis, tape worm and malaria very rare, toxoplasmosis uncommon, and so on. Where parasitic infestations are common, e.g. developing countries, it's related to sanitation, environmental prevention, animal management.