Most causes of death are because of the presence of associated anomalies and malformations, which are common for pulmonary agenesis especially involving right-sided defects. Those suffering right-sided defects normally have poorer prognosis than those with left-sided defects, partly because the right side of the lung is usually more prone to infection considering the standard anatomical position of right bronchus, and also because cases with right-sided lung disorders have shown higher association rates with other anomalies. In fact, it has been suggested that right-sided defects produce a more severe mediastinal shift, distorting the trachea and great vessels. Pulmonary agenesis was a confusing term before Boyden's classification was published. Before that, the term agenesis was used frequently when reporting cases related to underdeveloped lungs, regardless of the degree of the underdevelopment. In 1955, Boyden classified pulmonary agenesis into 3 groups: agenesis, aplasia, and hypoplasia, which has been adopted by most researchers nowadays. As a clarification, the term “pulmonary agenesis” in this Wikipedia page will only be referring to the agenesis in Boyden's classification, hence the information contained may not be valid for pulmonary aplasia or hypoplasia unless specifically mentioned.