This wouldn't work for viruses since they're using a fairly arbitrary coding which wouldn't exactly match up with viral DNA/RNA. But there are other genome synthesis methods which can and do produce viruses. However, that's been possible for many years now (viruses are extremely small so the expensive DNA synthesis methods are still cheap and doable) and no one's done it, despite worries about someone taking published genomes of the Spanish Influenza or smallpox and synthesizing them. Like biowarfare in general, I wonder if the reason is simply that it's harder than it looks and anyone smart enough to do it just isn't that evil?