A method of identifying a biomarker indicative of a subject mammal's condition, comprising the steps: a) selecting a first set of test mammals having a first disease; b) obtaining a first oral sample containing a first microbial community from each of the first set of test mammals having the first disease, wherein the first microbial community comprises one or more microbial types; c) treating each of the first set of test mammals having the first disease, who have been first oral sampled, so as to eliminate or reduce the first disease; d) obtaining a second oral sample containing a second microbial community from each of the first set of test mammals who have been treated, wherein the second microbial community comprises one or more microbial types; e) making the first disease reoccur in each of the first set of test mammals who have been second oral sampled; f) obtaining a third oral sample containing a third microbial community from each of the first set of test mammals in whom the first disease has reoccurred, wherein the third microbial community comprises one or more microbial types; g) measuring the first, second and third oral samples to obtain abundances of the one or more microbial types in the first, second and third microbial communities, respectively; h) statistically analyzing the obtained abundances of the one or more microbial types in the first, second and third microbial communities across the first set of test mammals to identify those microbial types whose abundances correlate with a statistical significance to a condition of the first set of test mammals as a first group of microbial types, wherein the condition is selected from the group consisting of: presence of the first disease, severity of the first disease, sensitivity to the first disease, and combinations thereof; i) selecting one or more microbial types from the first group of microbial types as the biomarker indicative of said subject mammal's condition. a) selecting a first set of test mammals having a first disease; b) obtaining a first oral sample containing a first microbial community from each of the first set of test mammals having the first disease, wherein the first microbial community comprises one or more microbial types; c) treating each of the first set of test mammals having the first disease, who have been first oral sampled, so as to eliminate or reduce the first disease; d) obtaining a second oral sample containing a second microbial community from each of the first set of test mammals who have been treated, wherein the second microbial community comprises one or more microbial types; e) making the first disease reoccur in each of the first set of test mammals who have been second oral sampled; f) obtaining a third oral sample containing a third microbial community from each of the first set of test mammals in whom the first disease has reoccurred, wherein the third microbial community comprises one or more microbial types; g) measuring the first, second and third oral samples to obtain abundances of the one or more microbial types in the first, second and third microbial communities, respectively; h) statistically analyzing the obtained abundances of the one or more microbial types in the first, second and third microbial communities across the first set of test mammals to identify those microbial types whose abundances correlate with a statistical significance to a condition of the first set of test mammals as a first group of microbial types, wherein the condition is selected from the group consisting of: presence of the first disease, severity of the first disease, sensitivity to the first disease, and combinations thereof; i) selecting one or more microbial types from the first group of microbial types as the biomarker indicative of said subject mammal's condition.