Cancer
Before World War II, doctors and hospitals were getting better at collecting (or getting and keeping) data about their patients who had cancer, but it was rare for this data to be shared with other doctors or hospitals. This changed after WWII, when medical research centers found out that different countries had very different number of cases of cancer. Because of this, many countries created national public health organizations (which studied public health issues in an entire country). These national public health organizations began to bring together health data from many different doctors and hospitals. This helped them figure out some of the reasons why cancer was so much more common in certain places. For example, in Japan, people studying cancer found out that people who had survived the atomic bombings of Hiroshima and Nagasaki had bone marrow that was completely destroyed. This helped them realize that diseased bone marrow could also be destroyed with radiation, which was a very important step in figuring out that leukemia (a blood cancer) can be treated with bone marrow transplants.