Final answer:
Patients with genital herpes are most contagious during an outbreak when blisters or sores are present.
Step-by-step explanation:
The nurse would explain to a patient with genital herpes that they are most contagious during an outbreak when blisters are present. Genital herpes, which is usually caused by herpes simplex virus type 2 (HSV-2), can result in painful blisters on the genital area. During the outbreak, the virus is present in the blisters, and transmission can occur through direct contact with these lesions. Even when blisters are not present, individuals with genital herpes can still shed the virus and potentially transmit it to others, but the highest risk of contagion is when the blisters are active and open.