What are the chances of getting a tapeworm from a few bites of a "very rare"/almost raw burger?

At a bbq yesterday, my burger was cooked on the outside but was still pretty raw inside, it was on the grill forever so I figured it was cooked. I ate a few bites before I realized, and didn't get sick, but what are the chances of contracting a tapeworm? I bought it at the local supermarket here in Boulder, I think my only symptoms so far are paranoia.