[theCoolScientist (OP)]

How do these viruses like Ebola(epidemic viruses) suddenly come out of nowhere and become threat to humanity? How do they stay dormant for such a long time?

[chiralSPO]

For many of these diseases, the virus (or bacterium, or other pathogen) lives in another animal (bird flu, swine flu, ebola from bats, etc.). This host species has already evolved in a way that it is no longer dying in droves because of the disease, merely carrying it, and possibly slightly sickened. This can last for long periods of time. Then, either due to a mutation in the pathogen, or chance exposure, the disease transfers to humans. If people have never been exposed to the pathogen before, they are unlikely to have a good immune response, and if the disease is easily communicable from person to person, it can take the form of an epidemic.

[evan_au]

These are called zoonotic diseases.
See http://en.wikipedia.org/wiki/Zoonosis