  The nonperturbative QCD vacuum at finite temperature and a finite baryon
density in an external magnetic field is studied. Equations relating
nonperturbative condensates to the thermodynamic pressure for $T\neq 0$, $\mu_q
\neq 0$ and $H\neq 0$ are obtained, and low-energy theorems are derived. A bulk
viscosity $\zeta(T, \mu, H)$ is expressed in terms of basic thermodynamical
quantities describing the quark-gluon matter at $T\neq 0$, $\mu_q \neq 0$, and
$H\neq 0$. Various limiting cases are also considered.
