A method of assessing whether a subject mammal has a target condition, comprising the steps: a) defining the target condition, wherein the target condition is selected from the group consisting of a disease, severity of a disease, sensitivity to a disease, and combinations thereof and the disease is a microbe-related disease; defining a first group of biomarkers each having a higher abundance in oral cavities of a set of test mammals with said target condition compared to oral cavities of a set of test mammals without said target condition, wherein the first group of biomarkers are bacterial genera selected from the group consisting of Leptotrichia, Prevotella, Fusobacterium, TM7, Porphyromonas, Tannerella, Selenomonas, Lachnospiraceae, Comamonadaceae, Peptococcus, Aggregatibacter, Catonella, Treponema, SRI, Campylobacter, Eubacterium, Peptostreptococcus, Bacteroidaceae, Solobacterium, Johnsonella, Oribacterium, Veillonellaceae, and combinations thereof; b) defining a second group of biomarkers each having a lower abundance in the oral cavities of the set of test mammals with said target condition compared to the oral cavities of the set of test mammals without said target condition, wherein the second group of biomarkers are bacterial genera selected from the group consisting of Streptococcus, Rothia, Actinomyces, Haemophilus, Lautropia, and combinations thereof.; c) formulating a linear function of the abundances of the first group of biomarkers and the abundances of the second group of biomarkers that is useful for assessing whether the subject mammal has the target condition comprising a formula: f Ai, Aj = b ∑ i ∈ N Ai N − ∑ j ∈ M Aj M where N is a total number of the biomarkers in the first group, M is a total number of the biomarkers in the second group, Ai is an abundance of each biomarker i in the first group, Aj is an abundance of each biomarker j in the second group, ∑ i ∈ N Ai is a sum of Ai over all biomarkers i in the first group, ∑ j ∈ M Aj is a sum of Aj over all biomarkers j in the second group, and b is a constant; d) obtaining a sample from an oral cavity of the subject mammal, wherein the obtained sample is capable of containing the first group of biomarkers and the second group of biomarkers; e) measuring abundances of the first group of biomarkers in the obtained sample from the subject mammal; f) measuring abundances of the second group of biomarkers in the obtained sample from the subject mammal; and g) inputting the measured abundances of the first group and the second group of biomarkers into the formulated function to assess whether the subject mammal has the target condition. a) defining the target condition, wherein the target condition is selected from the group consisting of a disease, severity of a disease, sensitivity to a disease, and combinations thereof and the disease is a microbe-related disease; defining a first group of biomarkers each having a higher abundance in oral cavities of a set of test mammals with said target condition compared to oral cavities of a set of test mammals without said target condition, wherein the first group of biomarkers are bacterial genera selected from the group consisting of Leptotrichia, Prevotella, Fusobacterium, TM7, Porphyromonas, Tannerella, Selenomonas, Lachnospiraceae, Comamonadaceae, Peptococcus, Aggregatibacter, Catonella, Treponema, SRI, Campylobacter, Eubacterium, Peptostreptococcus, Bacteroidaceae, Solobacterium, Johnsonella, Oribacterium, Veillonellaceae, and combinations thereof; b) defining a second group of biomarkers each having a lower abundance in the oral cavities of the set of test mammals with said target condition compared to the oral cavities of the set of test mammals without said target condition, wherein the second group of biomarkers are bacterial genera selected from the group consisting of Streptococcus, Rothia, Actinomyces, Haemophilus, Lautropia, and combinations thereof.; c) formulating a linear function of the abundances of the first group of biomarkers and the abundances of the second group of biomarkers that is useful for assessing whether the subject mammal has the target condition comprising a formula: f Ai, Aj = b ∑ i ∈ N Ai N − ∑ j ∈ M Aj M where N is a total number of the biomarkers in the first group, M is a total number of the biomarkers in the second group, Ai is an abundance of each biomarker i in the first group, Aj is an abundance of each biomarker j in the second group, ∑ i ∈ N Ai is a sum of Ai over all biomarkers i in the first group, ∑ j ∈ M Aj is a sum of Aj over all biomarkers j in the second group, and b is a constant; d) obtaining a sample from an oral cavity of the subject mammal, wherein the obtained sample is capable of containing the first group of biomarkers and the second group of biomarkers; e) measuring abundances of the first group of biomarkers in the obtained sample from the subject mammal; f) measuring abundances of the second group of biomarkers in the obtained sample from the subject mammal; and g) inputting the measured abundances of the first group and the second group of biomarkers into the formulated function to assess whether the subject mammal has the target condition.