  We observed the cluster Abell 2218 (z=0.175), with ISOCAM on board the
Infrared Space Observatory, using two filters with reference wavelengths of 6.7
and 14.3 microns. We detected 67 extragalactic mid-infrared sources. Using the
``GRASIL'' models of Silva et al. (1998) we have obtained acceptable and well
constrained fits to the observed spectral energy distributions (SEDs) of 41 of
these sources, for which additional optical/near-infrared magnitudes, and, in
some cases, redshifts, were available. We have then determined the total
infrared luminosities and star formation rates (SFRs) of these 41 sources, of
which 27 are cluster members and 14 are field galaxies at a median redshift
z~0.6.
  The SEDs of most of the ISOCAM cluster members are best fit by models with
negligible star formation in the last ~1 Gyr. The 7 faintest cluster members
have a slightly higher, but still very mild, star-formation activity. The
ISOCAM-selected cluster galaxies appear to be an unbiased subsample of the
overall cluster population. If A2218 is undergoing a merger, as suggested by
some optical and X-ray analyses, then this merger does not seem to affect the
mid-infrared properties of its galaxies.
  The SEDs of most ISOCAM-selected field sources are best fit by models with
significant ongoing (and recent) star formation, resembling those of massive
star-forming spirals or (post)starburst galaxies. Their median SFR is 22 solar
masses per year. Eight of the 14 field sources are Luminous IR Galaxies.
(Abridged)
