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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.
Polycythemia Vera. This slow-developing disease occurs when bone marrow produces too many red blood cells, which can lead to thickening of the blood. The condition is thought to be caused in part by a mutation in a gene called JAK2, which regulates the production of various types of blood cells, keeping them in proper balance.