 Syphilis is most commonly spread through sexual activity . It may also be transmitted from mother to baby during pregnancy or at birth , resulting in congenital syphilis . Other human diseases caused by related Treponema pallidum include yaws ( subspecies pertenue ) , pinta ( subspecies carateum ) , and bejel ( subspecies endemicum ) . Diagnosis is usually made by using blood tests ; the bacteria can also be detected using dark field microscopy . The Center for Disease Control recommends all pregnant women be tested . 
