Syphilis, caused by the bacteria Treponema Pallidum, can be a very serious infection that can increase your risk of contraction HIV by 3 to 5 times. If you have syphilis and HIV it is more likely that you could give HIV to someone else by having unprotected sex.
Having both syphilis and HIV can make the syphilis infection progress more quickly than others that have only syphilis. Also, having both diseases can make the treatment of syphilis more difficult.
As with any STD, it is transmitted by unprotected sex. The best way to keep STD free is to remain non sexually active