  We study meson-baryon scattering with strangeness -1 in unitary chiral
perturbation theory. Ten coupled channels are considered in our work, namely
$\pi^0 \Lambda$, $\pi^0 \Sigma^0$, $\pi^- \Sigma^+$, $\pi^+ \Sigma^-$, $K^- p$,
$\bar{K}^0 n$, $\eta \Lambda$, $\eta \Sigma^0$, $K^0 \Xi^0$ and $K^+ \Xi^-$. A
large amount of experimental data are analyzed, including the recent precise
measurement by the SIDDHARTA Collaboration of the energy shift and width of the
$1s$ state of kaonic hydrogen. This leads to a strong constraint on the free
parameters in our theory and of the resulting meson-baryon scattering
amplitudes. We also analyze the uncertainty that stems by using several
different strategies to perform the fits to data. It is found that large
uncertainties in the subthreshold extrapolation of the $K^-p$ scattering
amplitude arise by employing either only one common weak pseudoscalar decay
constant or distinguishing between $f_\pi$, $f_K$ and $f_\eta$. However, in
both cases a good reproduction of experimental data is obtained. We also
discuss the pole content of the resulting $S$-wave amplitudes, particularly in
connection with the two-pole structure of the $\Lambda(1405)$ resonance.
