  Basing on main principles of statistical mechanics only, an exact virial
expansion for path probability distribution of molecular Brownian particle in a
fluid is derived which connects response of the distribution to perturbations
of the fluid and statistical correlations of its molecules with Brownian
particle. The expansion implies that (i) spatial spread of these correlations
is finite, (ii) this is inconsistent with Gaussian distribution involved by the
``molecular chaos'' hypothesis, and (iii) real path distribution possesses
power-law long tails. This means that actual Brownian path never can be
disjointed into statistically independent fragments, even in the Boltzmann-Grad
gas, but behaves as if Brownian particle's diffusivity undergoes scaleless
low-frequency fluctuations.
