We are used to seeing massive growth rates with Android, but I am sure this is one statistic Google will not be touting in their quarterly reports.
Juniper Networks Mobile Threat Center reports Android malware samples had increased from 400 to 13,302 during the last 6 months of 2011, a growth rate of 3325%.
The worst offenders were Spyware, at 63%, but 36% could have cost you actual money, in the form of premium rate SMS Trojans.
Malware was also becoming more sophisticated, like Droid KungFu using encrypted payloads to avoid detection and Droid Dream disguising itself as a legitimate app.
While Juniper Networks found some malware on Windows Mobile (0.7% of the total) they did not identify any such threats on Windows Phone.
The full report can be seen here.