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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.
Quick Answer. Thick blood is caused by a variety of conditions, such as heavy proteins in the blood, too much blood in circulation or a blood clot system imbalance, according to Southeastern Medical Oncology Center. Diseases such as cancer and lupus can also lead to thick blood. Continue Reading.