This immunity is apparently not due to the absence of favourable conditions, but rather to the presence of some inimical factor which prevents the development of the parasite.
Enmann, who interprets the name as "she who prevents increase" (in contrast to Leto, who made women prolific), considers the main point of the myth to be Niobe's loss of her children.
It should be free from dirt - that is to say, free from clay or soft mud, for instance, which prevents the cement adhering to its particles, or again from sewage matter or any substance which will chemically destroy the matrix.
No, but it prevents the deaths of other innocents.
If he doesn't kill her, she might be what prevents him from building his Army of Souls.