  If known, the spectrum of heavy-hadron molecules will be a key tool to
disentangle the nature of the exotic states that are being discovered in
experiments. Here we argue that the general features of the molecular spectrum
can be deduced from the idea that the short-range interaction between the heavy
hadrons is effectively described by scalar and vector meson exchange, i.e. the
$\sigma$, $\omega$ and $\rho$ mesons. By means of a contact-range theory where
the couplings are saturated by the aforementioned light mesons we are indeed
able to postdict the $X(3872)$ (as a $D^* \bar{D}$ molecule) from the three
$P_c(4312)$, $P_c(4440)$ and $P_c(4457)$ pentaquarks (as $\bar{D}\Sigma_c$ and
$\bar{D}^* \Sigma_c$ molecules). We predict a $J^{PC} = 1^{--}$ $D \bar{D}_1$
molecule at $4240-4260\,{\rm MeV}$ which might support the hypothesis that the
$Y(4260)$ is at least partly molecular. The extension of these ideas to the
light baryons requires minor modifications, after which we recover approximate
SU(4)-Wigner symmetry in the two-nucleon system and approximately reproduce the
masses of the deuteron and the virtual state.
