The Goiânia accident [ɡoˈjɐ̃njə] was a radioactive contamination accident that occurred on September 13, 1987, in Goiânia, Goiás, Brazil, after an unsecured radiotherapy source was stolen from an abandoned hospital site in the city. It was subsequently handled by many people, resulting in four deaths. About 112,000 people were examined for radioactive contamination and 249 of them were found to have been contaminated.
When I first learned of this I found it remarkable that experts figured out what the likely cause was at all. I guess infectious disease experts have to be trained to recognize patterns of illness caused by radioactive material? Because primarily they would be looking for biological stuff.