Most Anti-Virus software packages are like flu shots.
- They only protect against the known or more popular virus strains.
- They are not replacements for other preventative actions.
- They are updated regularly and must be current to be protected.
- When there is an outbreak, it takes some time until the new virus can be identified and a vaccine is developed.
More advanced Anti-Virus software will also act as a disinfecting filter. As the computer is used on the network, it will keep viruses at bay by scrubbing the contents of the data (much like white blood cells, which react to newly discovered antigens).
On the other hand, there's also a lot to be said for the idea that they really ought to be unnecessary. Security is the Operating System's job. If a virus gets in, and can cause damage, then obviously the OS hasn't been doing its job.
Anti-virus software is a bandaid \1ix applied to make up for weaknesses in the OS' security model. All AV software really does is let the OS get away with its security failures, and make AV software manufacturers rich.