In the past, it was fairly simple to create read only recovery media for a computer: you just made sure to use write once disks. This meant that if you accidentally booted the machine to the normal OS, any malware you are trying to get rid of couldn't infect the recovery media. In times past, you might even receive a premade disk (or a set of them) from the manufacturer.
However, most modern computers don't even have a CD/DVD drive anymore, and the size of recovery media is fast making DVDs impractical for this purpose. USB drives are becoming a more common alternative for storing external recovery media.
However, unlike CDs and DVDs, USB drives do not typically have hardware level protection from writes. (There are some more expensive models that have such a feature.) In fact, USB drives are (or at least used to be) an extremely simple and common way of spreading malware between computers. This makes me concerned that if the machine boots to the normal OS instead of the back up (due to user error, the BIOS not recognizing it's bootable, etc.), the recovery media itself could very easily become infected.
Am I overly concerned, or is my concern legitimate? Even though I by no means regularly contract malware, I prefer to have the peace of mind that comes with a fairly reliable recovery plan.