A pandemic is a rapid spread of an infectious disease across a large region. An apocalyptic disease would combine incurability (like Ebola), lethality (like rabies), extreme infectiousness (like the common cold) and a long incubation period (like HIV/Aids). If such a virus spreads around the world before people become aware of the danger, the international health system would have to move with unprecedented speed and resources to save mankind.
Pandemics are likely to occur in the future, but their danger is very hard to estimate. So far, pandemics have received far more attention than other natural existential threats. A natural pandemic killing all humans is unlikely but plausible.
The majority of global pandemic threats are not existential risks. The estimates of this risk vary wildly; even expert commentators tend to be highly uncertain about the odds. Predictions also tend to lump together several different kinds of pandemics and thus can make the future sound much more dangerous than it really is. For example, a concerned expert may estimate that there is 50% chance of a global pandemic during the 21st century. Taken at face value this is extremely frightening, but with further scrutiny it becomes clear that the vast majority of that probability would be diseases with low death rates and certainly not pandemics that have a chance of killing all of humanity (Harack, 2016).
The complexity of pandemics is also the reason why straightforward extrapolations from history tend to be deeply flawed. Human civilization today is unprecedented in several relevant ways. We can’t study past examples of globe-spanning civilizations who actively tried to protect themselves from this danger. Knowledge of past pandemics is still incredibly valuable; it’s just very hard to meaningfully generalize it to our situation. (Harack, 2016)