The control of diseases spread by the fecal–oral route depends on the improvements in personal hygiene and sanitation that accompany general economic development. In contrast, efforts at preventing the spread of multihost parasites is usually focused on the simul-taneous treatment of infected humans and control or elimination of the nonhuman host.
To be effective, such measures must be applied in a comprehensive and coordinated manner over large areas. Administrative problems, political imbroglios, development of resistance in parasites and intermediate hosts, technical difficulties, and funding shortages have, individually and together, limited the success of such efforts. A case in point was the failure of the worldwide malaria eradication effort launched by the World Health Or-ganization in 1955. This has refocused attention on alternative control measures, includ-ing immunization. Until recently, the development of effective parasitic vaccines has been constrained by the complexities of their immunologic interactions with the human host. Monoclonal antibodies have helped identify antigens responsible for the induction of im-munity to a number of parasitic infections, including malaria, leishmaniasis, and schisto-somiasis. The subsequent cloning of the structural genes encoding such antigens has made a large-scale production of vaccine antigen feasible. It is further possible that the entire step of antigen production and purification could be bypassed by the use of syn-thetic peptide or anti-idiotype vaccines. All these approaches are currently being devel-oped. Malaria vaccines are undergoing clinical trials.