Today's teens may be more susceptible to herpes infections during sex than teens of just a few years ago. As if teens (and their parents) didn't already have enough to worry about.

The reason for the concern, according to a report by the Centers for Disease Control (CDC), is that it appears that fewer teens have antibodies to the type 1 (HSV-1) or type 2 (HSV-2) herpes simplex viruses that cause herpes.

Herpes infections can occur anywhere on the body, with the mouth and the genitals the two most common sites. It used to be that HSV-1 was responsible for most herpes infections of the lips and mouth, commonly appearing as cold sores (but not canker sores), while HSV-2 was the primary cause of genital herpes infections.

While oral sex does lessen many risks of sexually-transmitted diseases, HSV can easily be transmitted from the mouth to the genitals, even when cold sores are not present.

This has been changing. HSV-1 is now responsible for an ever-rising number of cases of genital herpes. One study found that over 70% of the cases of genital herpes in women under 30 were due to HSV-1.

According to the CDC, most individuals infected with HSV-1 or HSV-2 experience either no symptoms or have very mild symptoms that go unnoticed or are mistaken for another skin condition. Because of this, most people infected with HSV-2 are not aware of their infection. When symptoms do occur, they typically appear as one or more blisters on or around the genitals, rectum or mouth.

The blisters break and leave painful sores that may take two to four weeks to heal. Experiencing these symptoms is sometimes referred to as having an "outbreak." The first time someone has an outbreak they may also experience flu-like symptoms such as fever, body aches and swollen glands.

Antibodies, the body's protective response, to HSV-1 have been declining, particularly among teens. During the period from 1999 to 2004, almost 40% of teens aged 14-19 had antibodies to combat the herpes virus. In 2005-2010, that number dropped to 30%.

This means that teens are less protected from herpes infections as they begin to be sexually active than they were just a few years ago. It also makes it likely that genital herpes infections will start to rise.

Other findings of the study were that antibodies to HSV-1 also declined last decade among people in their 20s, though not to the degree that it did in teenagers. HSV-1 frequency in 30- to 40-year-olds did not change. Neither did HSV-2 frequency in any age group from 14 to 49.

The many safe sex campaigns of the last 30 years may have had the unintended consequence of making people think that oral sex is safe, which it certainly is not.

While oral sex does lessen many risks of sexually-transmitted diseases, HSV can easily be transmitted from the mouth to the genitals, even when cold sores are not present. This is how people infected with HSV, but who show no symptoms, can be completely unaware that they are transmitting the virus to their partners.

The study appears in the Journal of Infectious Diseases.