In such countries it is one of the classic diseases of childhood, with most cases occurring in children up to age 15;  most people become infected before adulthood, and 10% of young adults remain susceptible. In the United States, a temperate country, the Centers for Disease Control and Prevention (CDC) do not require state health departments to report infections of chickenpox, and only 31 states volunteered this information as of 2013. A 2013 study conducted by the social media disease surveillance tool called Sickweather used anecdotal reports of chickenpox infections on social media systems Facebook and Twitter  to measure and rank states with the most infections per capita, with Maryland, Tennessee and Illinois in the top three.In the tropics, chickenpox often occurs in older people and may cause more serious disease. In adults, the pock marks are darker and the scars more prominent than in children. Society and culture
Etymology
How the term chickenpox originated is not clear but it may be due to it being a relatively mild disease. It has been said to be derived from chickpeas, based on resemblance of the vesicles to chickpeas, or to come from the rash resembling chicken pecks. Other suggestions include the designation chicken for a child (i.e., literally child pox), a corruption of itching-pox, or the idea that the disease may have originated in chickens. Samuel Johnson explained the designation as "from its being of no very great danger".
U.S. National Library of Medicine.