What is the most dangerous disease out there?

Life: Life is a terminal disease. It is always 100% fatal. Moving on, there are things like ebola and meningiococcal disease which spread quickly and lead to very quick deaths, many cancers which spread relentlessly and are almost always fatal--but these are rare. Flu is common and kills a few. Not usually dangerous. It depends on what you are asking. Probably ebola wins, but obesity kills many more.

Dangerous Disease.: Are you referring to most virulent virus, fatal rate without or with treatment or most deaths historically? Dangerous infections include: Marburg Virus, Ebola Zaire, Hantavirus, H5N1 (Avian Flu), Junin Virus (Argentine Hemorrhagic Fever), Lassa Fever, Crimea – Congo Fever,Machupo virus (Bolivian hemorrhagic fever), Kyasanur Forest Virus (KFD), Rabies, Smallpox & Dengue fever, plague & anthrax.