One defence that is already in place occurs at the DNA printing level. Because of the cost of this equipment, most DNA printing is outsourced to third parties. These guys compare the DNA you send them to that of known pathogens like Anthrax and if your DNA matches they won't print it. There are many other people thinking about this space and what other safeguards we need, one of the main ones is the early detection system you talk about... with cheap DNA sequencing on a future iPhone maybe this will help!