A method of enriching microbial cell-free nucleic acid sequences in a sample obtained from a mammalian host, the method comprising: (a) providing a mammalian host sample which has previously been obtained from the mammalian host, wherein the mammalian host sample comprises mammalian host cell-free nucleic acids and microbial cell-free nucleic acids; (b) (i) isolating cell-free nucleic acid length intervals from the mammalian host sample, thereby enriching in the sample microbial cell-free nucleic acids in the length interval ranges of: i. less than about 120 base pairs, ii. from about 240 to about 280 base pairs, and/or iii. from about 425 to about 475 base pairs in length; or (ii) removing cell-free nucleic acid length intervals from the mammalian host sample, wherein the removed one or more length intervals are selected from the group consisting of about 180 base pairs, about 360 base pairs, about 540 base pairs, about 720 base pairs, and about 900 base pairs, thereby enriching in the sample microbial cell-free nucleic acids in the length interval ranges of: i. less than about 120 base pairs, ii. from about 240 to about 280 base pairs, and/or iii. from about 425 to about 475 base pairs in length. (a) providing a mammalian host sample which has previously been obtained from the mammalian host, wherein the mammalian host sample comprises mammalian host cell-free nucleic acids and microbial cell-free nucleic acids; (b) (i) isolating cell-free nucleic acid length intervals from the mammalian host sample, thereby enriching in the sample microbial cell-free nucleic acids in the length interval ranges of: i. less than about 120 base pairs, ii. from about 240 to about 280 base pairs, and/or iii. from about 425 to about 475 base pairs in length; or (ii) removing cell-free nucleic acid length intervals from the mammalian host sample, wherein the removed one or more length intervals are selected from the group consisting of about 180 base pairs, about 360 base pairs, about 540 base pairs, about 720 base pairs, and about 900 base pairs, thereby enriching in the sample microbial cell-free nucleic acids in the length interval ranges of: i. less than about 120 base pairs, ii. from about 240 to about 280 base pairs, and/or iii. from about 425 to about 475 base pairs in length. (i) isolating cell-free nucleic acid length intervals from the mammalian host sample, thereby enriching in the sample microbial cell-free nucleic acids in the length interval ranges of: i. less than about 120 base pairs, ii. from about 240 to about 280 base pairs, and/or iii. from about 425 to about 475 base pairs in length; or i. less than about 120 base pairs, ii. from about 240 to about 280 base pairs, and/or iii. from about 425 to about 475 base pairs in length; or (ii) removing cell-free nucleic acid length intervals from the mammalian host sample, wherein the removed one or more length intervals are selected from the group consisting of about 180 base pairs, about 360 base pairs, about 540 base pairs, about 720 base pairs, and about 900 base pairs, thereby enriching in the sample microbial cell-free nucleic acids in the length interval ranges of: i. less than about 120 base pairs, ii. from about 240 to about 280 base pairs, and/or iii. from about 425 to about 475 base pairs in length. i. less than about 120 base pairs, ii. from about 240 to about 280 base pairs, and/or iii. from about 425 to about 475 base pairs in length.