Though you've been trapped inside of a dark, lonely cave for thousands of years, you've been fortunate enough to infect a single human being by the name of Dave. Though he's clearly adventurous enough to go spelunking deep into the ground, Dave otherwise leads a pretty standard life (at least, for now). He lives alone in the suburbs of a large city, has an affinity for animals, and spends most of the day working in a comfortable office job.
Since you've already infected Dave, it would be easy enough to change him -- at any moment, your victim could find himself painfully doubling over, his body reforming and sprouting scales. But it would be much more effective to begin spreading yourself to other humans first, especially if things turned south for poor Dave.