Amazon's Fire TV and Fire TV Stick devices have been infected by a virus. The virus doesn't specifically target these devices, just anything running Android. And because these both do, it means they're at risk.
The virus is called ADB.Miner. It installs itself as an app called 'Test' under the innocuous-sounding package name 'com.google.time.timer'. Once it's infected your device, it starts mining cryptocurrency – your device will slow down, video playback will stop altogether and a notification saying 'Test' will appear randomly on-screen along with the green Android robot icon.
The good news? If you've never tinkered with your Fire TV device's developer options, you're not at risk. Those that have used their device to watch pirated films and TV shows via dodgy third-part apps are most at risk.
If you have been infected, you're advised to perform a factory reset.
The lesson is clear – steer clear of dodgy third-party apps and don't try and watch illegally-available content on your device. Stick to the trusted providers, and you should be golden.