I'm wondering why are chicken pox dangerous as an adult?

Outcome variability: At it's peak, chickenpox was most common in preschool kids, with > 2 million cases/yr , 50 deaths & 10, 000 hospitalizations. With the adoption of the vaccine the kid problem has become less an issue & sporadic adult cases have gotten more attention. Adults are at higher risk of a fatal pneumonia for reasons that are unexplained.We see a similar increased morbidity with mumps in adulthood.