Herpes simplex 1 virus and toxoplasmosis are common infections that that many of use have (65% and 10% of americans, respectively). Both of these are chronic, latent infections that can occasionally cause problems (i.e. in HIV patients) but most of the time don't.
Could there by many other such common, latent infections in the human population that we don't know of? Do we have any idea of how many there would be?