Herpes simplex virus (HSV) infection is caused by HSV-1 or HSV-2 and can cause oral or genital sores or blisters. It is transmitted through direct contact with an infected individual, even without visible sores. While HSV-1 typically causes oral herpes, it can also cause genital herpes. HSV infection is diagnosed through physical examination, viral culture, PCR testing of sores, or blood tests to detect antibodies.