Intensities of spectra of individual atoms or molecules typically vary linearly with the numerical gas density. However, if gas densities are sufficiently increased, quite generally contributions may also be observed that vary as density squared, cubed... These are the collision-induced spectra of two-body (and quite possibly three-body,...) collisional complexes. The collision-induced spectra have sometimes been separated from the continua of individual atoms and molecules, based on the characteristic density dependences. In other words, a virial expansion in terms of powers of the numerical gas density is often observable, just as this is widely known for the virial expansion of the equation of state of compressed gases. The first term of the expansion, which is linear in density, represents the ideal gas (or "ordinary) spectra where these exist. (This first term vanishes for the infrared inactive gases,) And the quadratic, cubic,... terms of the virial expansions arise from optical transitions of binary, ternary,... intermolecular complexes, which are (often unjustifyably) neglected in the ideal gas approximation of spectroscopy.