Any thumbdrive / USB flash drive / USB flash stick you found is not automatically danger-proof. There are known incidents whereby the manufacturers themselves have faulty systems that introduce the malware / viruses ... into the thumbdrive / USB flash drive / USB flash stick during the time of manufacturing the product. Treat all devices with suspicion. Disable the auto-run feature and always scan the device before using if it's not a device you trust and own. For more paranoid measures, regardless of any device, simply scan them (even your own trusted devices).