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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.
Excess Blood Cells. Certain diseases, namely blood cancers, thicken the blood because they lead to abnormally high levels of blood cells. While rare, one of the more common reasons for thick blood is called polycythemia vera (PV), in which the body produces excessive blood cells -- mostly too many red blood cells.