I've seen similar, and much worse, dust particles both in the air and on the lens show up in certain conditions. Particularly in my brother's Quarter Horse stable with natural light coming in at a low angle. The only pictures taken that day that showed the dust were the ones shot in that stable at that time of day. Nowhere else did we see the effect (which, as it happens, was virtually identical to what you're showing) but in the barn, yet when viewing the same scene with the naked eye none of the "spots" were apparent.

Besides, ghosts look more like this: