The phrase "mystery illness" means a health condition or disease that doctors and scientists cannot explain or understand fully. It is called a mystery because it is unknown what causes it or how to treat it effectively. Full definition
Broken bones in Mykonos, mystery illness in Madrid and bus accidents in Thailand, we've seen it all and know overseas medical travel insurance can literally be a lifesaver. (worldnomads.co.uk)
Buhari's mystery illness also comes as political parties and potential candidates jockey for position ahead of the next presidential election in 2019. (punchng.com)
Atis Muehlenbachs, an agency pathologist in the special unit that investigates unexplained mystery illnesses and deaths, wasn't sure what to make of the cell samples when he and his team received them in 2013. (washingtonpost.com)