An athlete who uses blood from another person for blood doping runs the risk of contracting a blood-borne disease because

pathogens can exist in the blood and be passed on through transfusions

Explanation:

Although the risk of transfusion-transmitted infections today is lower than before, the supply of blood transfusions remains subject to contamination with known and unidentified human pathogens. This can cause these pathogens to be transmitted from one person to another. That is, the risk of infections by blood transfusions has been reduced over the years, but not eliminated.

This indicates that athletes who use someone else's blood for blood doping are at risk of contracting a blood-borne disease because pathogens can exist in the blood and then transfusions can be passed on.

Pathogens can exist in blood and then can be passed the transfusions