  Chiral perturbation theory and its unitarized versions have played an
important role in our understanding of the low-energy strong interaction. Yet,
so far, such studies typically deal exclusively with perturbative or
nonperturbative channels. In this letter, we report on the first global study
of meson-baryon scattering up to one-loop order. It is shown that covariant
baryon chiral perturbation theory, including its unitarization for the negative
strangeness sector, can describe meson-baryon scattering data remarkably well.
This provides a highly non-trivial check on the validity of this important
low-energy effective field theory of QCD. We show that the $\bar{K}N$ related
quantities can be better described in comparison with those of lower-order
studies, and with reduced uncertainties due to the stringent constraints from
the $\pi N$ and $K N$ phase shifts. In particular, we find that the two-pole
structure of $\Lambda(1405)$ persists up to one-loop order reinforcing the
existence of two-pole structures in dynamically generated states.
