Zombie Hollow turns into blobs on death
Small and medium blobs split into small or medium blobs on death
Medium blobs self-replicate, and can do so through walls
So all it takes is a single zombie hollow anywhere in your reality bubble and you have a potential blob infestation at hand.
Eg. I had to kill around 150 blobs (at least according to blob glob remains) from a single hollow.
You can’t leave a single large/medium blob alive or they’ll self-replicate, and small blobs can still turn into medium blobs on death.