Technically, soon as you participate, you got influence. It depends on the scale you're looking at, but I am very sure that I myself are making a difference in my enviroment. I'm sort of the quiet, stable type, and I'm known for trying to put things into a broader perspective and thinking before acting; generally dealing with things in a relaxed, thoughtful way and I'm pretty sure I've had a positive effect on people and that I've made a difference in some people's lives (most of it positive, anyway
).
If you're looking for the greater scale, I go with Lurkette- there have been many individuals who've made a difference. Add Leonardo daVinci, van Gogh, Einstein, Rosa Parks to that list.
However, I do think that most people of great performance do not achieve what they do solely on their own. There's almost always a wife/husband/advisor/confidant/mentor around for guidance and support.