I recently posted about the online security wars, a trend which will continue to grow as spam filtering improves. The WSJ recently ran an interesting article about ad networks distributing spyware:

In May, a virus in a banner ad on tomshardware.com automatically switched visitors to a Web site that downloaded "malware" -- malicious software designed to attack a computer -- onto the visitor's computer. ScanSafe Inc., one of the first security firms to discover the virus, estimates the banner ad was on the site for at least 24 hours and infected 50,000 to 100,000 computers before Tom's Hardware removed it.

As traffic streams consolidate and ad networks improve in efficiency many people who get marginalized are going to get more insidious in their attempts to make money. This fear will further consolidate web traffic toward trusted brands and place a premium on central ad networks.