What does Marlow ironically call "the playful paw-strokes of the wilderness"?

Small sicknesses were often a prelude to more serious illnesses that the unexplored tropical wilderness has in store for them.  

Perhaps I had a little fever, too. One can't live with one's finger everlastingly on one's pulse. I had often 'a little fever,' or a little touch of other things—the playful paw-strokes of the wilderness, the preliminary trifling before the more serious onslaught which came in due course."