nMalware Count
Source: McAfee AVERT’s DATs
This is the total count of viruses.

The big leap in 98 was due a virus writer using a virus generator producing 14000+ variants.

Percent increase has diminished significantly.  More worms, more complex viruses, kiddies more likely using backdoor programs rather than writing viruses.  CodeRed could have been written by a single person.  But Nimda definitely written by a group.

Better heuristics and generic detection.  So, fewer variants are sent into AV companies to be counted.

Lastly, due to size and speed considerations, we continue to improve our database by consolidating our detections, so the database doesn't grow as quickly.