Amazon's Fire TV and Fire TV Stick, like many devices, lets you download apps from an app store and increase the amount of stuff you can do. Fire OS is also based on Android, which means it's possible to sideload unofficial/unapproved apps onto your device with a little bit of know-how. But that can be pretty risky, and people who've sideloaded piracy-enabling apps are finding those same apps are letting in some nasty malware that's using their device to mine cryptocurrency.
AFTVNews is reporting that the malware in question is a worm, though it isn't directly targeting Fire TV devices. Instead Amazon#'s streaming gizmos are at risk because of the fact they're running software based on Android. It's called ADB.miner, and installs itself onto devices as an app called 'Test' under the package name "com.google.time.timer". Once it's in place it'll starts taking over the device's hardware to mine cryptocurrency, slowing them down in the process, and trying to spread to other Android devices on the network. On top of this it'll end up disrupting video playback performance and eventually causes the Android robot to appear on screen with the label "Test".
The good news is anyone who stayed well away from the Fire TV's developer options is completely safe from the virus. So you don't need to worry just because you have one plugged into the back of your TV. If you have, well, it's not entirely clear which apps have been spreading the worm, only that apps designed to give people access to pirate content seem to be the culprits.
It is possible to check for the bug, however, with AFTV News recommending Total Commander from the Amazon app store. It'll let you see which apps have been installed, and if you see "Test" then your device has been infiltrated. Total Commander can uninstall the bug (AFTV News has a full walkthrough), but it's been recommended that users reset their devices to Factory Settings to undo any other changes the worm may have made to your device. It's a pain, I know, but it's better safe than sorry.
There's a third option, but that's restricted to people who know exactly what they're doing - and certainly not for amateurs. You can install a modified version of the malware (available on the XDA forums), which will update the initial installation and remove the mining capabilities. It's not the kind of thing just anyone should attempt, but some people have reported that it solved the issues after their initial attempts to remove the virus failed.