Until it decides to mutate from a bad bug to a super bad bug and we don't any way to stop it.
The 1918 bug was by far the most virulent in history.
It is, of course, possible that a much more virulent type than the 1918 version could evolve, but it seems unlikely.
There have actually been bugs with much higher mortality rates and speed of attack than the 1918 one. They tend to actually be LESS deadly, as they kill their hosts so fast the disease can’t spread far. What was so deadly about the 1918 flu was its balance of characteristics, which allowed it to spread rapidly while still being very deadly. Of course, modern high-speed transportation changes the equations.
BTW, my Dad used to talk about being six years old and watching his father and uncles carry eight family members out of a neighbor’s house. All had died.