Candida usually exists as a harmless fungus that grows in the vagina, anus, mouth, and under the foreskin in uncircumcised men.
However, when yeast grows in excess it causes irritation and unpleasant symptoms in these areas.
It’s not actually a STD. But you can pass it on by a variety of different behaviours including sex. This is why it’s included in the STD section.
In general, you don’t need to treat it unless you experience symptoms.