Answer:
Varicella-zoster virus
Explanation:
This type of virus is the one that is responsible for getting chickenpox, which are also called varicella and shingles. There is a big possibility that grandpa Davis had chicken pox when he was a little boy, maybe a teen, but the virus never left the body. On the contrary, it's hidden in the nerve tissue. The virus remains passive through one's life, but sometimes, it wakes up and reaches the nerve fibers and as a result an individual gets shingles.