  The van-der-Waals version of the second virial coefficient is not far from
being exact if the model parameters are appropriately chosen. It is shown how
the van-der-Waals resemblance originates from the interplay of thermal
averaging and superposition of scattering phase shift contributions. The
derivation of the two parameters from the quantum virial coefficient reveals a
fermion-boson symmetry in non-ideal quantum gases. Numerical details are worked
out for the Helium quantum gases.
