At what point did I say anything about it being too deadly that it can't spread? It's entirely feasible that something could be very deadly and very contagious at the same time.

However, with that combination unless the host reproduces at a crazy high rate the virus won't last long because it'll run out of hosts to infect quickly.

In epidemiology, an infection is said to be endemic in a specific population or populated place when that infection is constantly present, or maintained at a baseline level.

For an infection to persist for a long time it needs to have somewhere to live, if everyone dies it can't become an endemic condition.