"While chickenpox in adults is not likely to lead to your death, it is much more dangerous than if you get this common disease as a child. The dangers of chicken pox in adults is actually more than just being a bit more uncomfortable, if you get adult chicken pox then it is much worse than if you get it as a child. Catching this as an adult will cause you to be more prone to getting congestion and a high fever, and the itching will be much worse.
The big risk is that you are also that you will have a severely weakened immune system and you are more likely to end up with pneumonia. Almost twenty percent of people that get adult chicken pox will get pneumonia and ten percent will get shingles. Chickenpox, like so many other childhood diseases is much worse when contracted as an adult versus as a child. " 
KnowsWhy site says:
"The complications appear to be occurring more in adulthood. Chicken pox can cause encephalitis, or pneumonia or wheezing or rapid breathing after 3-4 days after the rash appearance for the first time. Though the percentage of adults that get affected with chicken pox are very less, the percent that die due to this disease among them will be about 55 percent and the percent undergoing hospitalization will be 33 percent.
Adults face the complications of chicken pox with a bacterial infection. The bacteria that can enhance the chicken pox complications in adults are those that infect soft tissues. Bacterial infections in the bone, pneumonia, at joints, and toxic shock syndrome will lead to further viral infection in adults. The other conditions in the adult that can result in causing chicken pox are brain infection, problems in bleeding and cerebella ataxia. Adults get easily affected when they come in contact with the people who live in the environment where there is easy transmission of chicken pox."