The virial equation $P{V}_m=RT\left(1+\frac{B}{V_m}+\frac{C}{V_m^2}+\dots \right)$ reduces to the ideal gas equation when

If all the virial coefficients beyond the first are zero, the bracket equals 1 and $P{V}_m=RT$. For a real gas the coefficients are non-zero but their contributions vanish as $V_m\to \infty$ (low pressure).