[signal]

I don't think I am qualified to answer this, but I think it depends. Maybe someone more knowledgeable will intervene....

All of these anti-(spy,virus,etc.) work off definition libraries. So if a scan yields an infection, there necessarily must be a detected 'signature' of this virus on yr comp which matches the virus definition library.

This implies that obscure, new, or novel (i.e. more sophisticated) viruses (malware) may be undetectable via AV scans. As a virus proliferates, it is logged, then added to libraries, then you download a definition update et voila you detect it.

A promising new method of scanning is heuristic scans... [quoting wikipedia: "In computer science, a heuristic is a technique designed to solve a problem that ignores whether the solution can be proven to be correct, but which usually produces a good solution or solves a simpler problem that contains or intersects with the solution of the more complex problem.

Heuristics are intended to gain computational performance or conceptual simplicity, potentially at the cost of accuracy or precision."]

So, I think a motivated hacker who is smart will stay ahead of detection in a manner similar to the bankrobbers of the 1800s: it was much easier to rob a bank and get away with it in the 1800s rather than nowadays: there is so much technology to put up with; big-brother type intrusion... and I think its sphere of influence will eventually encompass the cyber sphere as well.