There are several million virus definitions floating around the internet these days. No single virus software is able to catch all of them that your system may be exposed to.
Think of it this way. A computer is something like the human body to a degree. You may walk into a store or an auditorium. Someone in that public place may have the flu, you may not get the flu, but the person next to you will.
The standard laptop or desktop simply does not have the resources to retain the databases of virus definitions used by virus scanning software, much less the software to run the virus scans. That being said, it is a reasonable assumption that at some point in time, your system will be infected. All this is based on a number of factors, browsing habits, opening spam emails, attachments even from people you know whose systems may be infected, which then passes the virus along to you, to name a few.
In order to clean your system, we use multiple cloud based scanning tools that contain 99% of the known viruses to date. We also use several offline tools. These offline tools scan your operating system from “the outside”.This ensures removal of any viruses that are in the definition databases. We then follow up with several scanners that operate within your operating system.
Then we look for and remove software packages that may have been installed on your system which are a “breeding ground” for attracting viruses.