If something has a precedent, then either it has happened before or something very similar has happened before. A precedent is when a similar situation has preceded the current situation. People typically use precedent in an argument to claim that "if it happened before, then it can happen again".
Outlawing tobacco products probably wouldn't work because of the precedent of prohibition in the 1920s.
* You are arguing that it's possible for people to catch diseases from animals, but there is no precedent for that.
