Computers in developing regions often lack the Internet connectivity and network bandwidth necessary to consistently download and apply software updates and security patches. However, even unconnected computers contract viruses and malware through the sharing of USB flash drives and other removable media. This paper introduces FlashPatch, a system for distributing software updates to computers in such areas by having software updates "piggy-back" on the existing flow of flash drives in rural regions. FlashPatch requires no changes in user behavior once the software has been installed. We implemented a proof-of-concept FlashPatch prototype and evaluated it in a field trial in Ghana. We present data on the prevalence and spread of viruses at our study site and offer experimental evidence of FlashPatch's effectiveness from a nine-month field trial. We found that FlashPatch provided additional antivirus protection to 30% of the machines in our study without imposing any tangible burdens on the system owners.