But there could be sufficient numbers to spread in many different ways, right? Unless, they only inject one into your body. Silly, eh?
Graphine is just a carbon crystal. Sometimes those are "doped" with other elements in order to induce different properties, like they do with silicon for semiconductors. Graphine is not animated, it does not move on its own nor does it diffuse through the body any differently than any other material. If it did, it would eventually be completely expelled.
If something diffuses OUT of your body, it would do the same with someone else's body. It would not diffuse IN.
The material doesn't "spread", any more than gravel does.