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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.
When the number of red blood cells is too low (anemia), blood carries less oxygen, and fatigue and weakness develop. When the number of red blood cells is too high (polycythemia), blood can become too thick, which may cause the blood to clot more easily and increase the risk of heart attacks and strokes.