  We investigate the low-temperature phase of QCD and the crossover region with
two light flavors of quarks. The chiral expansion around the point $(T, m_q =
0)$ in the temperature vs. quark-mass plane indicates that a sharp real-time
excitation exists with the quantum numbers of the pion. We determine its
dispersion relation and test the applicability of the chiral expansion. The
time-dependent correlators are also analyzed using the Maximum Entropy Method
(MEM), yielding consistent results. Finally, we test the predictions of
ordinary chiral perturbation theory around the point $(T = 0, m_q = 0)$ for the
temperature dependence of static observables. Around the crossover temperature,
we find that all quantities considered depend only mildly on the quark mass in
the considered range 8MeV $\leq \bar{m}^{\bar{\text{MS}}} \leq$ 15MeV.
