A method for determining the relative abundance of total bacteria in a sample, the method comprising: providing a fecal sample from a subject before the subject receives chemotherapy; determining the relative abundance of total bacteria in the fecal sample; detecting in the fecal sample bacteria that correlate with developing a bloodstream infection after chemotherapy and calculating the relative abundances of bacteria that correlate with developing a bloodstream infection after chemotherapy; and detecting in the fecal sample bacteria that correlate with not developing a bloodstream infection after chemotherapy and calculating the relative abundances of bacteria that correlate with not developing a bloodstream infection after chemotherapy; wherein: (i) the detecting of comprises an assay that detects a member of family Erysipelotrichaceae, a member of genus Lactobacillus, a member of genus Eggerthella, a member of genus Veillonella, or a combination thereof; and/or (ii) the detecting of comprises an assay that detects a member of the order RF39 in the class Mollicutes, a member of the order Clostridiales, a member of family Barnesiellaceae, a member of family Coriobacteriaceae, a member of family Rikenellaceae, a member of genus Butyricimonas, a member of genus Faecalibacterium, such as Faecalibacterium prausnitzii, a member of genus Oscillospira, a member of family Christensenellaceae such as genus Christensenella, a member of genus Dehalobacterium, a member of genus Desulfovibrio, a member of genus Sutterella, a member of genus Methanobrevibacter, a member of genus Oxalobacter, or a combination thereof, wherein reference to a taxonomic group above species level excludes all lower taxonomic members of that group that are otherwise specifically identified. providing a fecal sample from a subject before the subject receives chemotherapy; determining the relative abundance of total bacteria in the fecal sample; detecting in the fecal sample bacteria that correlate with developing a bloodstream infection after chemotherapy and calculating the relative abundances of bacteria that correlate with developing a bloodstream infection after chemotherapy; and detecting in the fecal sample bacteria that correlate with not developing a bloodstream infection after chemotherapy and calculating the relative abundances of bacteria that correlate with not developing a bloodstream infection after chemotherapy; (i) the detecting of comprises an assay that detects a member of family Erysipelotrichaceae, a member of genus Lactobacillus, a member of genus Eggerthella, a member of genus Veillonella, or a combination thereof; and/or (ii) the detecting of comprises an assay that detects a member of the order RF39 in the class Mollicutes, a member of the order Clostridiales, a member of family Barnesiellaceae, a member of family Coriobacteriaceae, a member of family Rikenellaceae, a member of genus Butyricimonas, a member of genus Faecalibacterium, such as Faecalibacterium prausnitzii, a member of genus Oscillospira, a member of family Christensenellaceae such as genus Christensenella, a member of genus Dehalobacterium, a member of genus Desulfovibrio, a member of genus Sutterella, a member of genus Methanobrevibacter, a member of genus Oxalobacter, or a combination thereof,