What is this??? (Score 1)

Surfing the Web I have come across the http://www.infectedornot.com/ site, which includes two online scanners that apparently scan the PC in a very short time. They also claim to detect more malware than any other antivirus installed on the computer. Supposedly these tools can detect viruses running on the computer. I tried one of them and was actually quite surprised at how fast it was. It didn't detect anything unusual, but asked me to use the second scanner which, so it says, can detect anything malicious on my PC, active or not. I was surprised at the distinction made between active and latent malware. Is it that there are viruses on computers waiting for a specific moment or action to activate? Also, the same page includes statistics showing how many scanned computers were actually infected. Not only that, it says that (about 40% of computers, or something like that) many of these had an antivirus installed. This makes me wonder: if, despite having an up-to-date antivirus installed you still have viruses, then, what purpose does the antivirus serve? The vendor says that it detects over 700,000 viruses, is this true or is it an exaggeration? Thanks and bye!!!