Chicken pox is caused by the varicella virus. You are right that, in most people, chicken pox occurs only once. When someone is first infected by the varicella virus, they will break out in a blistering, crusting rash over most of their body - this is "chicken pox." Most of the time, the body's immune system will quickly bring the infection under control and the rash will go away. However, the virus is never totally eliminated from the virus - rather, it goes into hibernation, primarily living inside nerve cells along the spinal column.
Our immune systems, however, tend to keep it in check and keep it from reactivating.
As we age, however, our immune systems become less effective. Therefore in older adults or in people with weak immune systems, especially during times of stress or illness
, the virus sometimes escapes from control and reactivates. It doesn't cause 'chicken pox' however - the immune system is too strong still to allow that. Rather, it causes 'shingles' which is a localized infection of the skin and nerves in a particular pattern originating from nerve where the virus was hiding.
If you are an older adult, you should talk to your primary care doctor
about receiving the shingles vaccine, which is a relatively new vaccine that can be used to prevent this reactivation phenomenon.