  We present mid-infrared (2-12 micron) spectra of the microquasar SS 433
obtained with the Infrared Space Observatory (spectroscopic mode of ISOPHOT and
ISOCAM). We compare them to the spectra of four Wolf-Rayet stars: WR78, WR134,
WR136, and WR147 in the same wavelength range. The mid-infrared spectrum of SS
433 mainly shows HI and HeI emission lines and is very similar to the spectrum
of WR147, a WN8(h)+B0.5V binary. The 2-12 micron continuum emission of SS 433
corresponds to optically thin and partially optically thick free-free emission,
from which we calculate a mass loss rate of 2-3 x 10^{-4} Msun/yr if the wind
is homogeneous and a third of these values if it is clumped. This is consistent
with a strong stellar wind from a WN star. However, following recent studies
concluding that the mass donor star of SS 433 is not a Wolf-Rayet star, we
propose that this strong wind out flows from a geometrically thick envelope of
material that surrounds the compact object like a stellar atmosphere, imitating
the Wolf-Rayet phenomenon. This wind could also wrap the mass donor star, and
at larger distances (~ 40 AU), it might form a dust envelope from which the
thermal emission, detected with ISOPHOT at 25 micron and 60 micron, would
originate. This wind also probably feeds the material that is ejected in the
orbital plane of the binary system and that forms the equatorial outflow
detected in radio at distances > 100 AU.
