Severe bleeding is a critical marker of severe dengue infection because it signifies the progression to dengue hemorrhagic fever or dengue shock syndrome, both of which can be life-threatening. This complication results from increased vascular permeability, thrombocytopenia, and coagulopathy, leading to hemorrhagic manifestations. Unlike mild rash, low-grade fever, or headache, which are common and non-specific symptoms, severe bleeding indicates a breakdown in hemostatic mechanisms and requires urgent medical intervention to prevent mortality. Prompt recognition of bleeding is essential to manage fluid resuscitation and support care.

Reference: Harrison’s Principles of Internal Medicine, Kasper et al., 20th Edition.