I think it all depends on whatever mutations the virus can come up with to outsmart our procedures and vaccines.
It already figured out that killing most of its hosts led to no more hosts. So now, the death rate is %50.
Plus, it learned that if it could survive longer when expelled from the host, it could reach more hosts.
OR, it was engineered that way by USAMRIID.I doubt that even the USAMRIID could engineer something that virulent. But, what do I know?