Cases in the current outbreak have appeared primarily among children with asthma or a history of wheezing. The high incidence of the virus among children is unsurprising. Enteroviruses are common---causing between 10 and 15 million infections annually in the United States alone---and most adults have developed adaptive immune defenses against them through previous exposure. Infants, children and teens, however, often have not been previously exposed to the virus and thus represent a particularly vulnerable population. In part, the frequent occurrence of enterovirus infections is attributable to the diversity of ways the virus can spread, including through physical contact with infected individuals, contact with fomites (objects infected individuals have touched that now harbor the virus), and drinking contaminated water.
Symptoms of infection are often relatively mild (runny nose, sneezing, coughing, fever, aches, etc.) but can sometimes be so severe as to require hospitalization. Though a typically rare occurrence, enterovirus infections have been associated with paralysis. It is instances of paralysis, especially in Colorado, that have prompted worry regarding the current outbreak.
Image courtesy of CNN.