Please describe the most serious, fatal, and rare virus/disease?

Rabies: I am not sure what the most rare disease is, but the most fatal would have to be rabies. Rabies has essentially 100% fatality if untreated. Fortunately the incubation period is long, so if someone has been bitten by a rabid animal, there are treatments available to prevent the death that will otherwise inevitably occur. Even some of the most virulent forms of ebola are less deadly than rabies.

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.