Understanding what NOT to eat, what's your opinion?

Of course I know that alcohol is off limits because of fetal alcohol syndrome, but some of the other things I don't really understand. I get conflicting inforamtion on about everything I google about what you cannot have while pregnant. They say in Japan, women are encouraged to eat sushi while pregnant, in the States they say it will cause miscarriages. From what I gathered the reason you should not eat raw or undercooked meats or seafood is because of listeria, isn't that something you can get any time and not just when you're pregnant? Is it because if you get it while you're pregnant the baby may not be able to fight it off like you can? I love sushi and love my steaks medium, and want to know exactly what are the risks for the baby and myself. Or any other foods that could hurt my pregnancy. Any answers would be greatly appreciated, thanks!:)