Very easily within the range of possibility. I would have to be multiple such organisms/viruses etc, for a variety of reasons, but it is definitely a concern.
First off, a "major die-off" might involve "only" 50% of Earth's population, or around 4 billion people at the moment. A lot of folks would consider even a mere 100 million a big deal... ;-)
However, it might still only take a single strain to kill 99%+ of humans (truthfully I think the main "problem" would be infecting so many, since some live in quite isolated groups). This research done in Australia with mice and mouse pox was quite disturbing:
https://www.newscientist.com/article/dn311-killer-mousepox-virus-raises-bioterror-fears/
A virus that kills every one of its victims, by wiping out part of their immune system, has been accidentally created by an Australian research team. The virus, a modified mousepox, does not affect humans, but it is closely related to smallpox, raising fears that the technology could be used in biowarfare."Kills every one of its victims" = 100% mortality.
So, the recipe to effectively wipe out humanity is something along the lines of the above, somehow designed to trigger 90 days or so after the organism is infectious. Three months would be long enough for transmission to pretty much all non-hermits. After that, virtually all infected people die in short order...