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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.
Anything that damages the inner lining of a vein may cause DVT -- surgery, an injury, or your immune system. If your blood is thick or flows slowly, it's more likely to form a clot, especially in a vein that's already damaged.