We are all ghosts

In several respects

First, without touching someone, all other people are just sound or light (or smell). These are representations of the actual person. Sight is the main way of identifying things, I guess, so this is photons, which are waves and particles or both or whatever hitting our eyeballs. So, in this respect, other people are kind of unreal, like ghosts. Movies play with this concept: maybe people you see walking down the street are actually ghosts...

Secondly, a ghost could be like a vapor or something; not solid. We are not as solid as we think we are. We are to each other, but to small particles, we're like a fog, at best. So from a minute perspective, we're ghosts.

Third, ghosts are often felt, but not seen. People say they're taken over by the holy ghost, or feel the cold of a ghost moving through them. So maybe people feel other people when they are removed, or at some distance, from them (like ghosts).

Fourth, maybe the dead actually do affect us, like the dying breaths being the spirit of that person, or memories and telepathic reconstructions (e.g. dreams) entering our minds, keeping the "spirit" of a deceased person "alive."

Fifth, from the perspective of the future, after we're dead (which will last for eternity), all we were was dust in the wind (basically, a ghost, if that). Plus, the eternity preceding us, as well, may as well have considered us ghosts from the future. The ghosts of christmas past and x-mas future wish you a happy holiday :-)

Maybe a ghost is someone who has spontaneously combusted.