Chickenpox is usually a mild infection and serious problems are rare. The most common problem linked with chickenpox in children is a bacterial infection in the spots, causing the surrounding skin to become more red and sore. Your doctor may prescribe antibiotics to treat it. Some children may also develop a middle ear infection. Rarely, chickenpox can cause encephalitis (inflammation of your brain) or pneumonia (an infection of your lungs). Varicella pneumonia is the most common complication in adults and causes wheezing and rapid breathing 3-4days after the rash appears.
Attribution: BUPA