I think that 400 viruses are created per year, or maybe it is per month. Or maybe the 400 is wrong altogether (which is why I am asking). If so many viruses are created, how can the anti-virus programs keep up with them all? There must be some that slip through. With so many hackers out there, I'm surprised there aren't more dangerous and undetectable viruses out there. Well, I'm definately not complaining, I don't want my computer being taken down by viruses. If it is 400 a month, though, then that would be its 4800 a year (wow, that's a lot!)
You cant count how many viruses are created... Some are small, some are big, some arent released, some only go to a few computers, some target certain computers, etc... Im sure it would be much more than 400 though if you tried to find them all, maybe 4,000 a month.
anti-virus doesn't work on such a linear path. Anti virus look for certain combinations of code, or access to specific files. Since there are a finite number of languages that they can be written in at any one time, the software simply checks the program for potentially dangerous code, and if it finds some, flags it and brings it to your attention.
At the same time, it is true that anti-virus and firewall software are ALWAYS one step behind. The hole or mistake must be found before it can be fixed, and sadly most of these holes are exploited by viruses much sooner than the developer can find them themself.
Take Windows XP for example, when it first came out, it was declared 'hack proof' MS quickly took that back and released updates within 72hrs of distribution as there were reports of as many as 86 seperate backdoor and other type of hackable enterances into XP.... this all happened 12hrs before Windows XP was officially released, lol.
So basically these softwares are based on a set of rules, and rules can always, ALWAYS be broken. The idea is to cover as many basis as you can as to make them bent instead of broken, thus the destruction is minimal.
I think the big question here is not how many viruses are created but how many computers are affected. There are I bet thousands of viruses created every month, because any body with basic computer programming knowledge (which is a lot of people by the way) can create a virus. Since virus is something that is destructive it is easy to create one. I mean I bet there are a lot of people who can create distruvtive macro which could delete all the system files, which is also considered as a viurs... But the important thing is how many of those viruses can the 'evil people' spread around, and often it is just a few of those (when I mean few, I mean a few of those million viruses) that enter the general person's computer.
Like William said, rules can always be broken (easily, for some pieces of software), and that goes for every piece of software. So it's impossible to know how many viruses are created, but one thing might be certain, the more people learn programming, the viruses might significantly increase.