Shingles is caused by the varicella-zoster virus, which first enters the body as chickenpox (which nearly every adult over 40 had as a child) and never leaves. It stays dormant in sensory nerve roots, and in about one-third of us, reactivates later in life as shingles.
https://www.aarp.org/health/conditions-treatments/info-2019/shingles-eye.html

