  The interstellar medium is crucial to understanding the physics of active
galaxies and the coevolution between supermassive black holes and their host
galaxies. However, direct gas measurements are limited by sensitivity and other
uncertainties. Dust provides an efficient indirect probe of the total gas. We
apply this technique to a large sample of quasars, whose total gas content
would be prohibitively expensive to measure. We present a comprehensive study
of the full (1 to 500 micron) infrared spectral energy distributions of 87
redshift <0.5 quasars selected from the Palomar-Green sample, using photometric
measurements from 2MASS, WISE, and Herschel, combined with Spitzer mid-infrared
(5 to 40 micron) spectra. With a newly developed Bayesian Markov Chain Monte
Carlo fitting method, we decompose various overlapping contributions to the
integrated spectral energy distribution, including starlight, warm dust from
the torus, and cooler dust on galaxy scales. This procedure yields a robust
dust mass, which we use to infer the gas mass, using a gas-to-dust ratio
constrained by the host galaxy stellar mass. Most (90%) quasar hosts have gas
fractions similar to those of massive, star-forming galaxies, although a
minority (10%) seem genuinely gas-deficient, resembling present-day massive
early-type galaxies. This result indicates that "quasar mode" feedback does not
occur or is ineffective in the host galaxies of low-redshift quasars. We also
find that quasars can boost the interstellar radiation field and heat dust on
galactic scales. This cautions against the common practice of using the
far-infrared luminosity to estimate the host galaxy star formation rate.
