Prior to World War II, known cases of radiation poisoning were so rare that doctors had little to no experience studying or treating it. But with the development of A-bombs underway, scientists with the Manhattan Project began studying the effects of radiation on mice and other animals. You know, so they could treat the people they didn't immediately incinerate like a Sarah Connor nightmare. A doctor named Egon Lorentz found that mice receiving lethal doses of radiation could be saved with a donation of bone marrow from a healthy mouse (And hopefully prevent any possible rodent-Godzillas). Fortunately, with radiation killing off rodent bone marrow and humans' in pretty much the same way, doctors were able to adapt the bone marrow transplant (BMT) procedure for humans.
Today, a transplant of bone marrow stem cells from a healthy donor is still the most effective treatment for diseases like leukemia, multiple myeloma, and certain types of lymphoma. The important (at least for therapeutic reasons) part of bone marrow is that it contains hematopoietic stem cells or "blood stem cells" that can become marrow, platelets, white blood cells, or red blood cells as needed. Even now, researchers are finding ways to use these blood stem cells to treat a growing number of diseases in new and exciting ways. Cool, but just next time maybe don't invent Uncle Bob's Instant Genocide to do it.