Hello,
Welcome to Just Answer and thanks for your question. If the blood on the bandaid is still wet, and if the person who wore it is infected, and if the portion of your body that touches it has an opening in the skin, there is a very small possibility that would be infected with whatever that person had. However, I cannot imagine a prudent person touching a used bandaid on a public conveyance.
You would first need to know if the person who left the bandaid had an infection. If they didn't your chances would be zero. Also zero unless you touched infected wet blood with an injured finger so that blood exchange could occur. Even with needlesticks from confirmed HIV infected blood, health care workers are infected only a minority of the time. We can assume your odds of getting an infection are nothing to worry about.
Nurse Practitioner
Board Certified Nurse Practitioner, MS, RN. Private practice and hospitalist experience.