Parasites are not bacteria or viruses: they’re animals, from single-celled protozoa to large worms, that take up residence in one or more animal “hosts” and use them for both shelter and nourishment during parts of their life cycle. There are more than 50 that can be transmitted to people who eat fish raw or undercooked, a handful of which are relatively common, and may require surgery to remove. Thanks to their more complex biological organization, parasites are sensitive to freezing (bacteria generally aren’t). So there’s a simple rule for eliminating parasites in fish and shellfish: either cook the food to a minimum of 140°F/60°C, or prefreeze it. The U.S. FDA recommends freezing at –31°F/–35°C for 15 hours, or –10°F/–23°C for seven days, treatments that are not feasible in home freezers, which seldom dip below 0°F.