I use a rule of thumb (a heuristic to be formal) that an anti-virus program detects 99% of all computer viruses (provided its up to date, and the maker keeps the current computer virus definitions current). Which means only 1% of computer viruses undetected.
Adding another anti-virus (with the same heuristic) 99% detected, or 1% undetected of the 1% undetected.
- 1-anti-virus 1% undetected computer viruses or 0.01. (1 in 100)
- 2-anti-virus 1% of 1% undetected computer viruses or 0.0001. (1 in 10,000)
- 3-anti-virus 1% of 1% of 1% undetected computer viruses or 0.0000001 (1 in 1,000,000)
One could install more anti-virus products, but it becomes difficult with updating each one, and the interaction. One product detects an update file as a potential virus, but the three programs work well together.
But, one in a million is favorable probability for detecting any possible computer virus. Better odds are possible with more anti-virus products, but there is an increase in the difficulty of using more anti-virus products. A diminishing margin for returns, and while decrease in numerical probability, its implausible.
That does not preclude the possibility of a computer virus, but then using common sense...don't open unknown attachments, scan downloads for a possible computer virus, routine scans for computer viruses, regular backups of data, regular restore points, etc. I do other routine things, but the point is avoiding obvious blunders, and developing effective habits for computer use. Avoid laziness and laxness to avoid a computer virus, or for that matter spyware.
So far, the three anti-virus products, with defensive measures has avoided any computer virus, although they have detected and eradicated computer viruses.
