Not something I had come across before seeing this article in Lancet Infectious Diseases. It is a nasty necrosis of the tissue of the mouth and face, with an unknown cause, but malnutrition and a bacterial agent are thought to be involved. It is also known (note to any medical students reading this: this is a synonym!) as cancrum oris.
There has been a Noma Day, and the website contains information about the NoNoma Federation and its activities.
You can probably search Google for more information as well as I can (but if you use the synonym you will probably not find the restaurant in Copenhagen or the lighting company, or many other fascinating but irrelevant things…), but there is a useful page in the MedlinePlus Health Encyclopedia which my Google search did not find.