Homogeneity of the prevalence of a pathogen among hosts can be tested by means of a 2xn i contingency table with elements X ik and (N k 2X ik ) [59] . Different proportions (i.e. lack of homogeneity) will indicate a property of the pathogen that we will call selectivity. Selectivity will be measured by the Cramer's coefficient of contingency [59] of the contingency table. If x 2 i is the chisquared of the 2xn i table, the index of selectivity of the pathogen will be: Both of these indices range from zero to one, with zero meaning equal prevalence of the pathogen over hosts, or of pathogens over the same host, i.e. no selectivity for the pathogen or the host.