What are some of the most dangerous viral disease?

A few favorites:: Ebola, marburg, rabies, smallpox, and lassa fever, to name some of the viruses you'd least want to have.

A long list: Almost any virus in the right circumstances can cause severe illness and sometimes death. The obvious dangerous ones include things like rabies, ebola, Marburg virsus, West Nile, Chichungya, enchephalitis and a host of others. Some are more dangerous in the already sick or immunosuppressed such as Herpes and Ebstein=Barr virus.

Dangerous viruses: Are you referring to most virulent virus, fatal rate without or with treatment or most deaths historically? Dangerous viruses 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.