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

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.
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.