Furnishing full protection in a wireless world involves three types of code: (1) encryption algorithms to scramble data, (2) digital certificates to restrict access, and (3) antivirus software. Encryption, the most demanding of the three, follows a fairly simple equation: the larger the algorithm, the stronger the security, and the more CPU cycles needed. WAP-enabled telephones do not have the horsepower to handle the bulky security software designed for PCs. At this point all handheld devices, including PDAs, are vulnerable to any virus that comes along. It is worth noting that there are currently no known viruses that attack wireless gear, but as mobile IP gains popularity, it will become an increasingly attractive target. "It's conceivable one could have a worm virus similar to explore.zip that could spread to every person's device in a matter of a few seconds," says Nachenberg. 
Saunders, S. et al., Wireless IP: Ready or Not, Here It Comes, Data Commun., 28 (12), 42–68, 1999.