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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.
Tweet. Thickening of a blood vessel may occur for a number of reasons. It is usually a result of changes within the vessel wall causing it to become thicker than normal. Alternatively it may arise from an enlargement of the vessel as a whole due to dilatation or birth defects.