Cachexia is the severe wasting of muscle and fat seen in serious chronic illness, notably advanced cancer, heart failure, chronic obstructive pulmonary disease, and AIDS. Unlike simple starvation, it involves a metabolic derangement driven by inflammation: the body breaks down skeletal muscle even when calories are supplied, and appetite is suppressed. Patients lose weight, strength, and function, and the condition predicts poor outcomes. Nutritional support alone cannot reverse it, though adequate protein and energy help; treatment addresses the underlying disease and inflammatory drivers. Recognising cachexia early matters because intervention is most effective before wasting becomes profound.