  We have computed the 15 microns integrated galaxy light produced by ISOCAM
galaxies above a sensitivity limit of 50 microJy. It sets a lower limit to the
15 microns extragalactic background light of (2.4+/-0.5) nW/m^2/Hz. The
redshift distribution of the ISOCAM galaxies is inferred from the
spectroscopically complete sample of galaxies in the Hubble Deep Field North
(HDFN). We demonstrate that mid (MIR) and far (FIR) infrared luminosities
correlate for local star forming galaxies and that this correlation is
consistent with the radio-FIR one up to z ~ 1. From these correlations, about
75 % of the ISOCAM galaxies are found to belong to the class of luminous
infrared galaxies (L[IR] >= 10^11 Lsol), with star formation rates of the order
of ~ 100 Msol/yr. The cross-correlation with the deepest X-ray surveys from the
Chandra and XMM-Newton observatories in the HDFN and Lockman Hole respectively,
indicates that at most 20 % of the 15 microns integrated galaxy light is due to
active galactic nuclei (AGNs) unless a large population of AGNs was missed by
Chandra and XMM-Newton. The contribution of ISOCAM galaxies to the peak of the
cosmic infrared background (CIRB) at 140 microns was computed from the MIR-FIR
correlations for star forming galaxies and from the spectral energy
distribution of the Seyfert 2, NGC 1068, for AGNs. We find that the galaxies
unveiled by ISOCAM surveys are responsible for the bulk of the CIRB, i.e
(16+/-5) nW/m^2/Hz as compared to the (25+/-7) nW/m^2/Hz measured with the COBE
satellite, with less than 10 % due to AGNs. Since the CIRB contains most of the
light radiated over the history of star formation in the universe, this means
that a large fraction of present-day stars must have formed during a dusty
starburst event similar to those revealed by ISOCAM.
