In 1951, Henrietta Lacks, a poor woman with a middle-school education, made one of the greatest medical contributions ever. Her cells, taken from a cervical-cancer biopsy, became the first immortal human cell line—the cells reproduce infinitely in a lab. Although other immortal lines have since been established, Lacks's "HeLa" cells are the standard in labs around the world. Together they outweigh 100 Empire State Buildings and could circle the equator three times. This month, PopSci contributor Rebecca Skloot's book, The Immortal Life of Henrietta Lacks, tells the story behind the woman who revolutionized modern medicine. Here, five reasons we should all thank Henrietta Lacks.