A fluid sample extraction device (101, 200, 300), comprising: a housing (140, 304) comprising (i) a proximal portion (144, 148, 202) comprising a proximal fluid passage (160, 319) therethrough, (ii) a distal portion (146, 150, 204) comprising a distal fluid passage (170, 322) therethrough, and (iii) an internal fluid passage (158, 307) between the proximal portion and the distal portion, wherein the internal fluid passage comprises at least one internal wall (166, 306), a distal open end, and a central protrusion (174, 186, 318) extending from a bottom surface of the proximal portion, wherein the central protrusion comprises an opening in fluid communication with a proximal open end (160a, 320); and a porous medium (184, 302) within the internal fluid passage, between the distal open end and the proximal open end, wherein the porous medium is configured to move in a first direction from a first position to a second position toward the proximal open end (160a, 320) when fluid is drawn into the internal fluid passage (158, 307) from the distal open end, and to move in a second direction from the second position to the first position when the fluid is forced toward the distal open end, and wherein the porous medium and the at least one internal wall are configured to allow the porous medium to move within the internal fluid passage to: form a space between at least a portion of the porous medium and at least a portion of the at least one internal wall of the internal fluid passage when the porous medium moves in the first direction from the first position to the second position when the fluid is drawn from the distal open end toward the proximal open end, to allow the fluid in the internal fluid passage to flow around the porous medium to the proximal open end of the central protrusion of the housing; and close the space when the porous medium is moved in the second direction from the second position to the first position when the fluid is forced toward the distal open end, to force the fluid in the internal fluid passage to flow toward the distal open end of the housing, through the porous medium; wherein the porous medium is formed of a material that is configured to removably capture nucleic acids in the porous medium from the fluid when the fluid is forced through the porous medium. a housing (140, 304) comprising (i) a proximal portion (144, 148, 202) comprising a proximal fluid passage (160, 319) therethrough, (ii) a distal portion (146, 150, 204) comprising a distal fluid passage (170, 322) therethrough, and (iii) an internal fluid passage (158, 307) between the proximal portion and the distal portion, wherein the internal fluid passage comprises at least one internal wall (166, 306), a distal open end, and a central protrusion (174, 186, 318) extending from a bottom surface of the proximal portion, wherein the central protrusion comprises an opening in fluid communication with a proximal open end (160a, 320); and a porous medium (184, 302) within the internal fluid passage, between the distal open end and the proximal open end, wherein the porous medium is configured to move in a first direction from a first position to a second position toward the proximal open end (160a, 320) when fluid is drawn into the internal fluid passage (158, 307) from the distal open end, and to move in a second direction from the second position to the first position when the fluid is forced toward the distal open end, and wherein the porous medium and the at least one internal wall are configured to allow the porous medium to move within the internal fluid passage to: form a space between at least a portion of the porous medium and at least a portion of the at least one internal wall of the internal fluid passage when the porous medium moves in the first direction from the first position to the second position when the fluid is drawn from the distal open end toward the proximal open end, to allow the fluid in the internal fluid passage to flow around the porous medium to the proximal open end of the central protrusion of the housing; and close the space when the porous medium is moved in the second direction from the second position to the first position when the fluid is forced toward the distal open end, to force the fluid in the internal fluid passage to flow toward the distal open end of the housing, through the porous medium; wherein the porous medium is formed of a material that is configured to removably capture nucleic acids in the porous medium from the fluid when the fluid is forced through the porous medium. form a space between at least a portion of the porous medium and at least a portion of the at least one internal wall of the internal fluid passage when the porous medium moves in the first direction from the first position to the second position when the fluid is drawn from the distal open end toward the proximal open end, to allow the fluid in the internal fluid passage to flow around the porous medium to the proximal open end of the central protrusion of the housing; and close the space when the porous medium is moved in the second direction from the second position to the first position when the fluid is forced toward the distal open end, to force the fluid in the internal fluid passage to flow toward the distal open end of the housing, through the porous medium; wherein the porous medium is formed of a material that is configured to removably capture nucleic acids in the porous medium from the fluid when the fluid is forced through the porous medium.