|If it helps, think of a computer virus in the same terms as a flu virus. Every year they have to make up a set of flu vaccines for the current season. Much like that all of the anti virus manufacturers have to set up new rules and definitions for active infections. The same holds true for a corporation like Microsoft. |
The issue is that the people creating the new infections are exceptionally intelligent, and committed, as it takes a fair amount of work to find and get around holes in new software(usually). So the manufacturer will always have to patch their software, and will always be behind. The other two comments above are perfectly legit.