Medical issues are certainly cited most often, while safety is less of a factor. In Moscow's case, for instance, there are a lot of concerns regarding organized crime. In certain countries, the risk of crime is higher than in other countries. Some countries don't have a solid justice system, which helps us find out whether a person has a criminal record. In other countries, it's just the opposite.
On March 10th, 2011. See this statement in context.