Each person has a reaction to this view that defy everything we know about rats and the pejorative connotation the word have among western culture.
The legend:
Legend has it that Laxman, Karni Mata's stepson (or the son of one of her storytellers), drowned in a pond in Kapil Sarovar in Kolayat Tehsil while he was attempting to drink from it. Karni Mata implored Yama, the god of death, to revive him. First refusing, Yama eventually relented, permitting Laxman and all of Karni mata's male children to be reincarnated as rats.
The story behind rats at the temple is different according to some local folklore. According to this version, a 20,000 strong army deserted a nearby battle and came running to Deshnoke. Upon learning of the sin of desertion, punishable by death, Karni Mata spared their lives but turned them into rats, and offered the temple as a future place to stay. The army of soldiers expressed their gratitude and promised to serve Karni Mata evermore.
Eating food that has been nibbled on by the rats is considered to be a "high honor". If one of them is killed, it must be replaced with one made of solid silver."
Nobody ever got a disease in these temples in India.
Worshipping Rats In India’s Karni Mata Temple. Photo via Fulvio Spada