  Simple pure luminosity evolution (PLE) models, in which galaxies brighten at
high redshift due to increased star-formation rates (SFRs), are known to
provide a good fit to the colours and number counts of galaxies throughout the
optical and near-infrared. We show that optically defined PLE models, where
dust reradiates absorbed optical light into infrared spectra composed of local
galaxy templates, fit galaxy counts and colours out to 8um and to at least
z=2.5. At 24-70um, the model is able to reproduce the observed source counts
with reasonable success if 16% of spiral galaxies show an excess in mid-IR flux
due to a warmer dust component and a higher SFR, in line with observations of
local starburst galaxies. There remains an under-prediction of the number of
faint-flux, high-z sources at 24um, so we explore how the evolution may be
altered to correct this. At 160um and longer wavelengths, the model fails, with
our model of normal galaxies accounting for only a few percent of sources in
these bands. However, we show that a PLE model of obscured AGN, which we have
previously shown to give a good fit to observations at 850um, also provides a
reasonable fit to the Herschel/BLAST number counts and redshift distributions
at 250-500um. In the context of a LCDM cosmology, an AGN contribution at
250-870um would remove the need to invoke a top-heavy IMF for high-redshift
starburst galaxies, although the excellent fit of the galaxy PLE model at
shorter wavelengths would still need to be explained.
