It doesn't need to be lethal, just highly transmissible. And persistent. Such as Herpes. And not necessarily even symptomatic.

Here's the requisite quote from Echopraxia, by Peter Watts:

> Even DNA computers, custom-built for a specific task and then tramped carelessly into wild genotypes like muddy footprints on a pristine floor. Nowadays it seemed like half the technical data on the planet were being stored genetically. Try sequencing a lung fluke and it was even money whether the base pairs you read would code for protein or the technical specs on the Denver sewer system.