According to some experts, Disease X could result in 20 times more fatalities than the coronavirus pandemic. Scientists believe Disease X could be caused by a zoonotic virus – a virus that jumps from animals to humans. The WHO believes Disease X is likely to emerge in tropical, low or middle-income countries – especially those already impacted by climate change. Ebola virus disease and Marburg virus disease: There are five strains of Ebola, as well as Marburg, which is a strikingly similar virus. Disease X: An unknown, hypothetical disease.