  We present new Spitzer IRS spectroscopy of Cygnus A, one of the most luminous
radio sources in the local universe. Data on the inner 20" are combined with
new reductions of MIPS and IRAC photometry as well as data from the literature
to form a radio through mid-infrared spectral energy distribution (SED). This
SED is then modeled as a combination of torus reprocessed active galactic
nucleus (AGN) radiation, dust enshrouded starburst, and a synchrotron jet. This
combination of physically motivated components successfully reproduces the
observed emission over almost 5 dex in frequency. The bolometric AGN luminosity
is found to be 10^12 L_\odot (90% of LIR), with a clumpy AGN-heated dust medium
extending to \sim130 pc from the supermassive black hole. Evidence is seen for
a break or cutoff in the core synchrotron emission. The associated population
of relativistic electrons could in principle be responsible for some of the
observed X-ray emission though the synchrotron self-Compton mechanism. The SED
requires a cool dust component, consistent with dust-reprocessed radiation from
ongoing star formation. Star formation contributes at least 6 \times 10^10
L_\odot to the bolometric output of Cygnus A, corresponding to a star formation
rate of \sim10 M_\odot yr-1.
