Midnight Mass Explained
4. The "Miracles"
The major catalyst for the religious fervour which sweeps Crockett Island are various "miracles" that which the devout believe have graced their community.
The first is when Leeza (Annarah Cymone), who was paralysed in a hunting accident, is able to walk again. This is followed by Mildred's de-aging alongside other ailments being cured amongst the rest of the community.
But these miraculous recoveries have nothing to do with God. Rather, these supernatural occurrences are due to the vampire blood they've been drinking regularly at church.
As Sarah explains, the vampire blood acts like an aggressive virus. When ingested, it repairs and heals the body at rapid speed. It's the cause for all the "miracles", but it's also the reason Erin lost her baby.
When enough of the blood is in someone, they'll reach a point of no return and transform into a vampire (like Puritt). However, stop consuming the blood before this critical point and it will fade from a body's system.
This is what happens to Leeza at the end when she tells Warren that she "can't feel [her] legs" anymore. The blood has dissipated from her body, leaving her how she was before her "miracle".