Comment by benrapscallion

The virus lies dormant in spinal and cranial ganglia for decades. If it reactivates, the immune system suppresses it, in the meantime. Past age 50-60, immunity wanes and these reactivations cannot be suppressed which allows the virus to travel back to the corresponding skin region as the spine segment to cause shingles. Presumably, after this the virus can also cross the blood brain barrier to cause neurodegeneration.