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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.
Conditions that can trigger excessive blood clotting in the heart and brain: 1  Atherosclerosis is a disease in which a waxy substance called plaque builds up inside your arteries. 2  Vasculitis is a disorder that causes the bodyâs blood vessels to become inflamed.