How can dengue hemorrhagic fever cause low bp?

Dengue: There are many mechanisms by which this could occur, including severe anemia induced by the intravascular hemolysis, renal failure, vascular collapse, dehydration and other mechanisms. You need to get a medical education to understand this fully, and don't think that this is place in which to obtain that. Good luck.

Hemorrhagic Fever: Agree with Dr. Raff. Dengue hemorrhagic fever can cause massive bleeding, hypotension, shock & death .