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The extra red blood cells make your blood thicker than normal. As a result, blood clots can form more easily. These clots can block blood flow through your arteries and veins, which can cause a heart attack or stroke. Thicker blood also doesn't flow as quickly to your body as normal blood.
Certain ethnic races (particularly Caucasian) may have a genetic predisposition towards thick blood. Defects in the coagulation genes adversely affect the capability of the body to turn off the switch that activates the clotting process. Thick blood may also be caused by the presence of harmful pathogens such as fungi, viruses, bacteria, and parasites.