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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.
Sudden unexplained bruising or blood spots under the skin or a sudden increase in the frequency of bruising may be caused by: 1  A medicine, such as aspirin or blood thinners (anticoagulants). 2  Infection that causes the buildup of toxin in the blood or tissues (sepsis).ruises develop when small blood vessels under the skin tear or rupture, most often from a bump or fall. Blood leaks into tissues under the skin and causes the black-and-blue color.