Sure. In Cohen's famous result, he says that a perfect virus detector should emit an alarm if and only if the input program can ever act like a virus (i.e., infect your machine and do damage).
Consider the following program:
f() is some harmless function, and
infect_and_do_damage() is a viral payload that infects your machine and does all sorts of damage (wipes your hard disk, steals all your money, whatever).
Let's consider what a perfect virus detector should say about this program:
f() can return, this is a virus and the virus detector should emit an alarm.
On the other hand, if
f() always enters an infinite loop and never returns, then the second line is dead code,
infect_and_do_damage() will never be invoked, this program will never act like a virus, and the virus detector should not set off any alarms.
So, the problem of determining whether this code is a virus is equivalent to the problem of determining whether the function
f() can ever halt. That's the famous halting problem, which is known to be undecidable.
In other words, detecting whether a program is a virus is at least as hard as detecting whether a program will halt. Thus, both problems are undecidable.
Note that this is a purely theoretical result. Undecidability is a purely theoretical construct. The fact that a problem is undecidable is not the end of the conversation; it is merely the beginning of the conversation.
In practice, there are various ways to attempt to deal with undecidability: e.g., try to write a solution that is probabilistically correct, even if it is not always correct on all programs; try to find a solution that works for the set of programs you're likely to find in practice, even if it doesn't work on all programs; allow the solution to occasionally answer "I don't know" or to err on the side of declaring a program a virus (or err on the side of false negatives); and so on.
So you should not treat this as a definitive statement that virus detection is impossible -- just because the problem is undecidable doesn't mean it is necessarily impossible to find a good-enough solution in practice. But it does identify some fundamental barriers to building a perfect virus detector.