Interestingly there’s research on mice that showed when mice fasted for 36 hours their immune cells went into bone marrow and were somehow optimized so they increased immunity, however don’t try it while building immunity as fasting potentially interferes with the origination of immune cells in the growth centers.
They exist in some iteration. I looked into it a while back.
Sedona, AZ : https://sedonawellnessretreat.com/water-fasting/
Dallas, TX: https://siddhayatan.org/spiritualretreats/waterfastretreat/
Boca Raton, FL: http://www.zendenyogaschool.com/intensive-detox-cleanse-retreats