My Win8.1 PC reported a virus detected in the Bitcoin database today. I just marked it as "Allowed" and then told it to ignore the Bitcoin directory there-after. If I'd picked the default action of "Quarantine" instead, I imagine I'd have been able to unquarantine it later, or else just download the block again. As it is, not only am I fine, but I can re-broadcast the block to anyone else who needs it. As long as one person has a copy, we're fine. The crypto means the block can't be forged. No coins are going to be lost.
It's mostly just to fuck with people who are too stupid to configure their antivirus properly.