Presently I think most viruses can't survive a reformat/reinstall... save the old ones that did nasty things like reset "unbreakable" passwords (eg. an old Apple virus that turned an expensive printer in to an equally expensive paperweight by setting its normally unset "usage" password - the "I forgot my password" solution was to buy a new printer).
Originally posted here by politroyx
For me, the most deadly virus is when the creator of the virus doesn't even know how to create the antidote for the virus he made.
MSN and Windows ME. I have Windows ME on my newest computer (about 1.5 years old) and got about 30 viruses (removed with AV programs before they could cause trouble) and about 25 trojans like Back Orifice 2000, Sub7 (almost all versions), NetBus Pro, Doly Trojan, Deep Throat that left my Windows ME so that Norton Auto-Protect wouldn't even let me start Windows so I had to take me computer into a technician.
I'm going to go by personal experience, and not by AV company satistics.
in short my answer is Nimda.
--- Long version alert ---
The only virus that has managed to cause any sort of concern in the environment that I manage is Nimda.
We had a moron in one of our departments that installed Kaaza onto his non-restricted workstation (couple that with a modem and you're asking for trouble)
After managing to install nimda on his workstation, he then went and managed to infect his departmental web server (after this all computers connecting to the web servers were infected)
this managed to propogate via network share, and served web pages to loads of 1st and 3rd party windows NT servers and workstations, and only came to our attention when my own part of the mess we call a network got hit (all killed by my AV protection).
The ensuing investigation found well over 5,000 infections, and identified 100 or so microsoft servers that had not been kept up to the recommended patch level by the Indian company we pay to do such things. (in fairness our own Microsoft guys screwed up too)
A brief history of infection
i cant speak from experience since i have never had virus toubles of any kind on my pc.
but i think that just as with biological viruses the best defense is education and prevention.
these viruses wouldnt replicate and spread as easily is the general public were well informed about how a virus works, and took preventative measures against this.
this is why HIV has gone down so much in america, people know about it and protect themselves.
this is why HIV spreads out of control in africa where people still dont know how to protect themselves.
it may seem like a strange comparison but thats just what i see.