Dualism is a dominant theory of life that considers reality to be a balance between two independent and fundamental principles: good and evil, mind and matter, nature and nurture. In the same manner we see the thread of dualism run through the ecology of parasitism: they can generate diversity but cause extinction, they may castrate a host but increase its growth rate, and they can stimulate an immune response but at the same time encourage a secondary chronic infection. Parasites inhabit individual hosts that are distributed as discrete patches, much like a metapopulation but these hosts are also nested within a spatially structured metapopulation and these within a meta-community of competent hosts. They often.
