Whats the risk of infection or transmission of disease during a transvaginal ultrasound ??

Same as in any...: The risk of catching something is the same as in any other medical office visit... meaning there could be another patient in the waiting room with cold or flu symptoms, and thus cough or sneeze viruses into the air that others in the same waiting room end up breathing in. Thus, wear a mask if worried. Also, avoid catching germs in restrooms. Ultrasound scans themselves don't transmit diseases.