HAPE is a severe and potentially fatal altitude illness caused by exaggerated hypoxic pulmonary vasoconstriction. This increases pulmonary artery pressure, leading to capillary stress failure and fluid leakage into the lungs. Symptoms include persistent cough, breathlessness, and cyanosis. Immediate descent, oxygen therapy, and medications like nifedipine are critical treatments, while gradual ascent and avoiding overexertion are key prevention strategies.