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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.
Excessive, thick phlegm or chest mucus can often be caused by viral or bacterial infections such as influenza, bronchitis and pneumonia, as well as irritants such as those inhaled during smoking. Chemotherapy also causes the immune system to overreact and excessive phlegm is often the result.