2B where B is the bandwidth of the signal. See page 272 in the Proakis text. The copies of the original spectrum appear centered at multiples of Fs. The convolution operation results in the spectral copies. In this case, the frequency of the digitized version of the signal is lower than the original analog version.
Some systems use aliasing in a beneficial way.
For example, a baseband signal can be recovered from a modulated signal through the process of downconversion, which can be implemented using aliasing. Fs This relation is true, irrespective of any aliasing. Analog signals at higher frequencies can't be appreciated as such in the discrete domain. The spectrum outside this range contains redundant information, due to sampling. Hence it is not considered to be computationally feasible.