Source: https://www.healthline.com/health/shingles-contagious#risk-factors
It's likely that your dad has had chicken pox in the past before.
This is because shingles is caused by reactivation of the varicella zoster virus.
Antiviral medicines such as acyclovir can help to treat shingles and shorten the length and severity of illness. They need to be taken as soon as possible to be the most effective.
The only way to reduce the risk of developing shingles is to get vaccinated. The CDC currently recommends that healthy adults 50 years and older get 2 doses of the shingles vaccine called Shingrix.
