As with most foods, eating sushi has some health risks. However, most can be minimized with proper preparation. Some large fish, such as tuna (especially bluefin), can contain high levels of mercury. Tuna can cause mercury poisoning when consumed in very large quantities over time. Parasite infection by raw fish is not common in the modern world (less than 40 cases per year in the US). Infections can generally be avoided by boiling, burning, preserving in salt or vinegar, or freezing to a certain temperature. Although "nigirizushi" will almost always appear in a raw form, often much of the fish has been previously frozen to specific temperatures to prevent parasites.