Since this is a real basic question I will answer in very basic
terms..... For more info you need an immunology text book...
>What does it take to be immune to a virus?
exposure to the virus or parts of the virus (Abs only)
>How long does the immunity last?
This varies tremendously but many immune responses if large enough or
seen enough times can last for decades (please no flames about
persistance of Ag here)
Can you become immune to the Ebola virus?
Yes, those who live to tell the tale had an effective immune response and
are probably immune to rechallange. The problem is it killed the rest
before they could kill it.