Herpes simplex virus 2 (HSV-2) is sexually transmitted. Symptoms include genital ulcers or sores. In addition to oral and genital sores, the virus can also lead to complications such as infection of the lining of the brain and the brain itself (meningoencephalitis) in neonatal infants due to infection during birth. However, some people have HSV-2 but do not show symptoms. Up to 30% of U.S. adults have antibodies against HSV-2. Cross-infection of type 1 and 2 viruses may occur from oral-genital contact.
More info about herpes simplex:
http://www.medhelp.org/medical-information/show/766/Herpes-simplex
Syphilis is an easily spread infection caused by the bacteria Treponema pallidum.
More information about syphilis:
http://www.medhelp.org/medical-information/show/510/Syphilis