Final answer:
Syphilis is an STI that can be fatal if left untreated, while other STIs may cause complications but are not typically fatal.
Step-by-step explanation:
Syphilis is an STI that can be fatal if left untreated. It is caused by the bacterium Treponema pallidum and can cause serious damage to the heart, brain, and other organs. If syphilis is not treated, it may eventually lead to death.
Other STIs, such as chlamydia, gonorrhea, and genital herpes, can cause complications and serious health issues if left untreated but are not typically fatal.