At the present time more is known about barriers to transmission of infectious agents between species than barriers to transmission within the same species. However differences in resistance to infection have been well-established within given species of various plants and domestic farm animals. Unsurprisingly several similar mechanisms have been observed in humans. A well-known human example of genetic protection is resistance to malaria in endemic areas which has been associated with polymorphism in alpha and beta chain globulin genes, cytoskeleton proteins, and protein/receptors on the surface of red blood cells. Studies regarding infection by Schistosoma mansoni show that the extent of infection depends largely on each individual's intrinsic resistance under the control of a single major gene which has now been located on q31-33 locus of the long arm of chromosome 5. This locus harbors several genes involved in differentiation of auxiliary T lymphocytes. With regard to HIV infection it has been known for several years that a small but significant number of individuals are relatively resistant. This resistance has been attributed to deletion of the gene coding for the chemokine receptor used by the virus as a co-receptor to infect macrophage.