An allergic explosion is a medical condition in which an individual experiences a sudden and severe reaction to an allergen. The term 'ACHOO' is an acronym for the medical term Allergic Contact Hyper-Sensitization Onset, which is used to describe the onset of an allergic reaction. This reaction can cause symptoms such as sneezing, coughing, wheezing, and difficulty breathing. In extreme cases, it can lead to anaphylaxis, a life-threatening condition that requires immediate medical attention.