Infant illness is a term that refers to any type of illness that affects infants. Two common infant illnesses are COLIC and CROUP. COLIC is a condition in which an infant cries for long periods of time, often for no apparent reason. CROUP is an infection of the upper airway that causes a barking cough and difficulty breathing. Both conditions can be distressing for parents and caregivers, but can usually be treated with medication or other therapies.