  Using a new color-color diagnostic diagram in the mid infrared built from
WISE data, the MIRDD, we compare narrow emission-line galaxies (NELGs) that
exhibit different activity types (star-forming galaxies, SFGs, and AGNs,
i.e.,LINERs, Sy2s and TOs), with broad-line AGNs (QSOs and Sy1s) and BL Lac
objects at low redshift ($z \le 0.25$). We show that the BL Lac objects occupy
in the MIRDD the same region as the LINERs, whereas the QSOs and Sy1s occupy an
intermediate region, between the LINERs and the Sy2s.In the MIRDD these
galaxies trace a sequence that can be reproduced by a power law, $F_\nu =
\nu^{\alpha}$, where the spectral index, $\alpha$, varies from 0 to $-2$, which
is similar to what is observed in the optical-ultraviolet part of the spectra
of AGNs with different luminosities.
  For the NELGs, we perform a stellar population synthesis analysis,
demonstrating that the ${\rm W}2-{\rm W}3$ color is tightly correlated with the
level of star formation in their host galaxies. A comparison of their MIR
colors with the colors yielded by energy distributions (SEDs) of galaxies with
different activity types, shows that the SED of the LINERs is similar to the
SEDs of the QSOs and Sy1s, consistent with AGN galaxies with mild star
formation, whereas the SEDs of the Sy2s and TOs are consistent with AGN
galaxies with strong star formation components. For the BL Lac objects, we can
only fit a SED that has no star formation component, consistent with AGNs in
elliptical-type galaxies.
  From their similarities in MIR colors and SEDs, we infer that, in the nearby
universe, the level of star formation activity most probably increases in the
host galaxies of emission-line galaxies with different activity types along the
sequence BL
Lac$\rightarrow$LINER$\rightarrow$QSO/Sy1$\rightarrow$Sy2$\rightarrow$TO$\rightarrow$SFG.
