Blood-borne pathogens are defined as

• A. Pathogenic microorganisms that are present in human blood and can cause disease
• B. Organisms present in insects that can be transferred to humans
• C. Microorganisms that travel from animal to animal, causing disease
• D. Pathogenic microorganisms that present in human blood, but that don't cause disease

Answer: (A)

Blood-borne pathogens are the pathogenic microorganisms that are found in human blood and have the potential to cause disease. This definition focuses on two parts: they must be in human blood, and they must be capable of causing illness. That’s why the statement describing organisms present in human blood that can cause disease is the correct one. It isn’t about organisms in insects (which would be vectors) or about diseases spreading from animal to animal (which describes zoonoses or cross-species transmission). It also isn’t correct to say they’re in human blood but don’t cause disease, because that would mean they aren’t pathogens. Real-world examples include viruses like hepatitis B and C, and HIV, which can be transmitted through blood and cause serious disease.