There are more than hundred antivirus softwares. A particutlar antivirus being the best for you depends on some conditions:
1. Are you connected to internet?
2. Do you have a fast processor and a high capacity ram?
3. Do you run a computer centre(do to many people access your computer daily)?

An antivirus operates by getting the checksum of all the known viruses. These are compiled as definition files used by the antivirus program. Thus whenever the operating system tries to open a file, it's first sent to the antivirus program for scanning. This is called realtime scanning. Some Antiviruses also make use of heuristics and other intelligent technologies. These work by checking for packed executables and typical virus behaviours. This is why some antiviruses detect programs that require exclusive access to the system as viruses.  It also means that a virus can infect your system if it has not been inspected by an antivirus company. This is exactly why I said no antivirus is good.

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