> How about a computer virus? Let it have a long latency period before it
> flares into destructive mode, so that it spreads everywhere. Then let
> the people be such that they can't live without computers. Maybe they
> are cyborgs or something. When the virus flares, they all die.
Only if they are a species made up exclusively of identical idiots. In any modestly diverse society (like ours) some idividuals will choose not to become cyborgs. Those who do will use many different operating systems, with many different anti-virus schemes (so no one virus can infect them all). Some of them will be paranoid, and insist on special hardware-level protections for their essential functions (which renders them pretty much immune to software attacks). Others will go for maximum enhancement, relying on the best availible defensive technolgies to spot pesky viruses (and other sneaky software) and keep them out.
With all these defenses, it is hard to believe that a killer computer virus could take out even a single planet. For an interstellar civilization the problem is much worse, because now you need an incubation time of centuries. So, your virus needs to remain undetected and functional through dozens of revolutionary changes - new operating systems, new hardware architectures, new approaches to virus protection, and who knows what else. Even then, all it takes to spoil the plan is for one person, anywhere, to notice the sleeping time bomb and alert his local antivirus-software dealer. I think we can safely rule out that possibility.
> You could probably do the same thing with a biological virus, except
> without nanotech it is harder to create something with the necessary
Physical viruses suffer from many of the same problems as the software variety. You want a virus that is 100% lethal (which they never are), affects 100% of the population (even the ones who have radically altered their genetic structures), can incubate for centuries before becoming active (how?), can not be cured by medical technology hundreds of years more advanced that what it was created by, and is never detected by anyone in all that time (in an age where biowarfare is no more exotic than the normal variety). Does this really sound like a plausible scenario to you?
Billy Brown, MCSE+I