Well, targeting and individual is probably not feasible for the reasons stated above: the large number of genes required to specify a single person and the mutation problem.
But constructing a target virus with a less ambitious specificity (say, against everyone with AB-negative blood type) is a lot easier to imagine that a space elevator. A space elevator requires an undiscovered superstrong material and a ginormous construction effort. For a designer virus the two main hurtles are assembling the virus and determining what proteins should make up the shell and what genes should go inside. The latter would probably be done by first using a cell simulator program to get a rough design and then using series of mutation/selection steps in the lab to increase the specificity and potency. We don’t have anything like a realistic cell simulator, but the simulation of protein and DNA interactions is an active area of research and I don’t doubt that there will be something good enough for this task within a few decades.