I work for an AV company and we definately dont write any viruses. We do however get targeted by a lot of virus writers. What they some how dont realise is that every file that comes through our mail gateway gets stripped off and goes straight into the virus lab to be analysed.
We will normally see viruses before the majority of users for these reasons:

1. Virus writers target AV companies
2. Some virus writers, write proof of concept viruses and therefore send to AV companies to make them aware (like whitehat hackers)
3. Users that are turned on and find something strange on there system, will normally call there AV supporter and send in a sample of the file
4. We also (starngely enough) have a very strong group of individuals who troll the newsgroups for new viruses (normally trojans) and send them into us
5. Lastly, all of the AV companies share all the viruses that they have. The process involves sending a catalogue of all viruses found that month to each other AV company. This changes when a particularily nasty little buggar surfaces, and this is of course sent to all AV companies as soon as analysis starts on it.

The vast majority of viruses are windows based. Near enough all of them are 'in the zoo', and not 'in the wild'. We detect over 86,000 viruses, about 200 ish are in the wild.

Keep Safe