Answer: shingles, varicella-zoster virus
Explanation:
It should be noted that shingles usually occur with old people and it causes a painful rash when the chicken pox virus is reactivated in the body.
Since the man is old and has painful fluid-filled vesicles that have begun crusting over in a wide band across the left side of his lumbar area and also has a history of chickenpox, it's very likely it is shingles.
