Y'all are obviously too young to know the symptoms of a stroke.
Like raw milk.
This is a story about a zombie apocalypse. Like kickap00 correctly pointed out, it would have been extremely lame for that to be the way she died.
The signs were too explicit for it to just coincidentally not be a stroke. What I think (added onto the theory that Eko is a carrier for the virus, thus possibly being the point of contagion to Miya) is that the virus is what caused the stroke. Because yes, this is a zombie apocalypse manga.
But that's too coincidental to not be a death by stroke. It may be an additional defense mechanism from the virus as insurance that the host's body will die and can be taken over, or it may be that everyone who's bitten and/or infected (and isn't immune to the virus) dies to a stroke.
We know biting is a point of contagion in most zombie stories, and it is in this one too. There's this underlying assumption that someone bitten just eventually kind of turns into a zombie, though. In reality, it would make much more sense for the virus to eventually cause a stroke - and freely take over the host body since it's now dead - than some non-descript "turn" mechanism. The virus can't take over living bodies, so it has an explicit mechanism for killing them in case they didn't die of blood loss.