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Thick blood may also be caused by the presence of harmful pathogens such as fungi, viruses, bacteria, and parasites. These pathogens can actually activate a coagulation response in the body as a way to avoid being attacked by the bodyâs immune system.
Polycythemia vera is a blood cancer that begins in the marrow of your bones, the soft center where new blood cells grow. With polycythemia vera, your marrow makes too many red blood cells, which causes your blood to get too thick. That can make you more likely to have blood clots, a stroke, or a heart attack.