Reusing a Marinade? Boil It First
A marinade that raw meat, poultry, or seafood has sat in picks up bacteria from that food's surface, the same way a cutting board would, so brushing it back onto the cooked food or serving it as a sauce as-is can recontaminate something that's already safe to eat. If you want to reuse it (as a basting liquid or finishing sauce), bring the whole batch to a full rolling boil for at least a minute first, which kills off the bacteria it picked up.
Sources & methodology: These tips draw on established food-science references —
Harold McGee's food-science writing, America's Test Kitchen /
Cook's Illustrated testing, Serious Eats'
technique reporting, USDA food safety guidance,
and peer-reviewed food-safety research (including UC Davis
cutting-board studies) — not reworded recipe-blog consensus.