Anti-virus is considered as a necessary security mechanism for the low-hanging fruit, predictable malware, most of it old, easy to detect, and still dangerous. Anti-virus primarily uses two methods to detect malware:
Signature: This method looks for known patterns of malware
Heuristics: In this method the behavior of potential malware is analyzed for malicious actions
Depending on the sophistication of the threat, and if detected, the solution may be able to "clean" the virus from the system. With encoding and encryption methods the norm for malware and hackers, detection is near impossible.
Note
A common method to exploit systems with malware is to bypass anti-virus using simple techniques. Methods include encoding, encryption, obfuscation, and random language compiling, all of which confuse anti-virus and the malware goes undetected. A quick search on the Internet will provide several sources on methods to evade and bypass anti-virus on a system. One example is the method provided...