A virus customized to use a higher concentration of materials than what exists in nature has a significantly lower chance of mutating into anything that could be destructive let alone functional outside its specialized environment compared to a normal virus that constantly reinvents itself to infect hosts and replicate as fast and efficiently as possible.
In general the more specialized they make this virus the lower the chances are for it to be remotely functional outside its special tailored environment.
Not that the zombie comments and theories aren't amusing. I do rather enjoy L4D, RE, etc. myself.
But all those stories are typically based on viruses that were already dangerous to humans and engineered to be weapons-grade effective etc.
Not something that started as harmless and was engineered to work in a non biological setting. It would have a better chance of becoming a "replicator" (stargate) than anything else and adapt to consume other materials besides the metals it was selected for.
Which may cause something like this:
http/www.youtube.com/watch?v=_0dYPnui3rM