A co-structure study is a comparison of demographic and/or genetic structure between
two or more species. Such a comparative analysis among a parasite and its host(s)
or among multiple parasite species is useful to elucidate factors that shape genetic
variation within and among parasite populations. I provide a brief review of how co-structure
studies in parasite systems can be used to address ecological, evolutionary, and epidemiological
questions. Subjects that can be addressed with parasite costructure studies range
from broad-scale analyses that compare phylogeographical patterns to local scale analyses
that examine among host transmission within a host population.