For someone in her condition the best treatment is drugs that suppress the immune system, and these do not make such a person an ideal candidate for spending time in a slum where tuberculosis is practically epidemic. But one night Ms. Boo tripped over an unabridged dictionary in her own apartment, puncturing a lung and breaking three ribs, and decided home wasn’t much safer. “I thought if I don’t work, I’m risking my mental health,” she said.

For as long as she has been a writer, Ms. Boo has only wanted to write about the poor and the disadvantaged. In 2000, while at The Washington Post, she won a Pulitzer Prize for a series about the mistreatment of the mentally retarded in the Washington area. “I think I grew up with a healthy respect for volatility, all the things you can’t control,” she said. “And I became aware of the ways in which people who write about the disadvantaged often underestimate its psychological contours, the uncertainty — economic or whatever.”

Ms. Boo is herself both a late bloomer and a prodigy. She grew up in and around Washington, where her parents, both Minnesotans, moved when her father became an aide to Representative Eugene McCarthy. (The family name is Swedish, an Americanized version of Bö.) After high school, by her own account a “confused late adolescent,” Ms. Boo took the civil service exam and became a clerk typist for the General Services Administration. When she discovered she was ill, she quit and stayed at home for a while, just reading, and then went to night school while typing again, this time for the Federal Election Commission.

Ms. Boo graduated from Barnard in the late ’80s, still typing — for The Columbia Daily Spectator, for which she wrote editorials — and was hired by Jack Shafer, then the editor of the Washington City Paper. Mr. Shafer, now a columnist for Reuters, said recently that he was impressed less by her writing than by her voluminous reading and her ability to think on her feet, and was amazed by how accomplished her first article was. “She had the soul of a poet but the arm strength of an investigative reporter,” he recalled.

Soon afterward he made Ms. Boo his No. 2, responsible not just for writing but also for editing the work of others, and from there she moved up the ladder to The Washington Monthly and then The Post, where she became known for the way she combined investigative digging, on-the-street reporting and brilliant writing. But she was never comfortable with interviewing official sources, she said, and is still proud she has never had lunch with one.

Another thing that makes her uncomfortable is policy wonkery, and by design “Beautiful Forevers,” a book as depressing as it is memorable, has no summing-up chapter full of recommendations. “I respect the division of labor,” she said. “My job is to lay it out clearly, not to give my policy prescriptions.” She added: “Very little journalism is world changing. But if change is to happen, it will be because people with power have a better sense of what’s happening to people who have none.”