I suppose that there are every day new viruses detected, that it is hard for the software companies to keep up. As a consequence there has to be a significant amount of any kind of malware, to which I simply refer to as viruses, to stay undetectable for a while. Will someone ruffly estimate or give numbers of the amount of those being undetected, at any given time? Since virus programmers have potentially more time and enthusiasm then their counterparts, who work for money, but may have more sophisticated methods it might come a nonlinear developement.
edit: you can leave the viruses that never get discovered out of scope. i just wonder how unprotected you are even with good antivirus software.