Can STDs spread when using sex toys? If so, how?

Yes! Fluids are fluids are fluids! When fluids are exchanged, (which happens if you have a sex toy on one vulva or inside a vagina and move it onto another vulva or into another vagina), that’s fluid bonding. So, unless you’re fluid bonded, I recommend washing them or using a different condom for each person. Most STIs are transmitted through fluids or blood.