Most of the computers harboring the Flashback botnet are in the United States and Canada, pretending to be a Mac software update or a Java updater.
More than 56 percent of the infected computers are in the U.S., according to Dr. Web, a Russian antivirus vendor.
Chitika regularly mines its ad impression data for trends in operating system and browser usage patterns.
A CNBC survey estimates that 55 million homes have at least one Apple gadget, and one in 10 of the non-Apple households plans to join them in the next year.
A new version of a well-known family of Mac malware exploits vulnerabilities in Java to steal usernames and passwords for online payment, banking, and credit card websites.
Increasingly, virus-writers are aiming at Mac users, such as the Flashback Java Trojan variant hitting systems running OS X 10.6 Snow Leopard.
Last year's sales of iPhones, iPads, and iPods topped nearly three decades of Macintosh products, according to analysts.
When software developers insisted on bringing PCs running Unix into the company, the network equipment maker had to respond.