Having a compromised immune system can cause an infestation of Demodex. They are supposed to be comensal, however I don't believe it any more, nor do the hundreds of people on Facebook's multiple support groups who live a nightmare, thanks to Demodex.

I had dry eye and used Xdemvy drops, which killed them. They moved to my scalp, so I used a tea tree oil shampoo and then poured 100% tea tree oil onto my scalp (wearing goggles to protect my eyes) and put on a shower cap, waited 10 minutes and washed it out. TTO kills Demodex in 3 minutes but I wasn't taking any chances. It really helped.

My immune system was compromised from breast cancer. What most people in the support groups say is that their doctors either don't know about Demodex, or don't want to deal with it. The only doctors who understand them are ophthalmologists, because that's the usual reason for blepharitis. My own dermatologist thinks they are good for us. Yikes. It was my ophthalmologist for dry eye who told me my chemo caused their proliferation.

Demodex may not be your issue, but studies have shown that 100% of cadavers have Demodex and 100% of people over 70 have them. It's only infants who don't. Good luck!

Note: Xdemvy worked so well for me that I bought stock in the company.