  We show that relativistic fluids behave as non-Newtonian fluids. First, we
discuss the problem of acausal propagation in the diffusion equation and
introduce the modified Maxwell-Cattaneo-Vernotte (MCV) equation. By using the
modified MCV equation, we obtain the causal dissipative relativistic (CDR)
fluid dynamics, where unphysical propagation with infinite velocity does not
exist. We further show that the problems of the violation of causality and
instability are intimately related, and the relativistic Navier-Stokes equation
is inadequate as the theory of relativistic fluids. Finally, the new
microscopic formula to calculate the transport coefficients of the CDR fluid
dynamics is discussed. The result of the microscopic formula is consistent with
that of the Boltzmann equation, i.e., Grad's moment method.
