Allergy is a disorder of the immune system often also referred to as atopy. Allergic reactions occur to environmental substances known as allergens; these reactions are acquired, predictable and rapid. Strictly, allergy is one of four forms of hypersensitivity and is called type I (or immediate) hypersensitivity.

Hives (urticaria) is a harmless type of rash or skin allergy that is usually caused by an allergy. The rash consists of circular, raised welts on the skin that are usually itchy and occur in batches.

Candida may cause yeast mouth infections (also known as thrush) in children with reduced immune function or in children taking certain antibiotics. Antibiotics may upset the balance of microorganisms in the body and allow an overgrowth of Candida.

Pneumonia is an inflammation of the lungs most often caused by infection with bacteria or a virus. Pneumonia can make it hard to breathe because the lungs have to work harder to get enough oxygen into the bloodstream.