Sexually transmitted disease
Syphilis can cause many serious problems if it is not treated.  It makes a person much more likely to get HIV, because it causes sores on the genitals that make it easier for HIV (and other STDs) to get spread.  After a few years, if people with syphilis do not get treated, they can have serious problems with the organs in their body.  Eventually - without treatment - syphilis infects the brain and causes death.