There's several ways that one can use to approach this:
1. Lycanthropy is a curse. So since curses are supernatural or magical in nature, this is completely up to the DM. If having any part of the werewolf causes a scratch, or puncture, then the curse is transmitted.
2. Lycanthropy is biological. Now we have to think about how lycanthropy is transmitted. Is it transmitted via teeth or nails that are hollow and essentially a saliva / poison is administered? If so, then it becomes a matter of time before the poison in the teeth has completely dried out that a scratch from the teeth mark is just that, a scratch.
3. Lycanthropy is viral / bacterial. This follows under biological, but merits its own discussion. Viruses and bacteria can be more resilient to continue living if the environment is right. For example, the AIDS virus has dozens of mutagenic variations and each medication cocktail designed to mitigate a particular strain will allow it to mutate into a new strain. So again, this can be whatever the DM wants when it comes to lycanthropy when it is viral or bacterial in nature.
Ultimately, as the other posters have stated, it's up to the DM, but if you want to put in the thought exercise, here's some ideas to get you going. I'm sure the others can jump in pull some cool ideas too.
Happy Gaming!