Lookout says it has seen more unique strains of Android malware in the past month than it did in all of last year. One strain seen earlier this year, called DroidDream, was downloaded more than 260,000 times before Google removed it, though additional variants keep appearing.

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Lookout says about 100 apps have been removed from the Android Market so far, a figure Google didn't dispute.

Malicious applications often masquerade as legitimate ones, such as games, calculators or pornographic photos and videos. They can appear in advertising links inside other applications. Their moneymaking schemes include new approaches that are impossible on PCs.

One scheme: Setting up fake quizzes charged to victims’ phone bills

One recent malicious app secretly subscribed victims up to a service that sends quizzes via text message. The pay service was charged to the victims' phone bills, which is presumably how the criminals got paid. They may have created the service or been hired by the creator to sign people up. Since malware can intercept text messages, it's likely the victims never saw the messages - just the charges.

A different piece of malware logs a person's incoming text messages and replies to them with spam and malicious links. Most mobile malware, however, keep their intentions hidden. Some apps set up a connection between the phone and a server under a criminal's control, which is used to send instructions.

Google points out that Android security features are designed to limit the interaction between applications and a user's data, and developers can be blocked. Users also are guilty of blithely click through warnings about what personal information an application will access.

Malicious programs for the iPhone have been rare. In large part, that's because Apple requires that it examine each application before it goes online. Still, the recent security incidents underline the threat even to the most seemingly secure devices.