But can't a virus be created to corrupt nodes, and over time spread to 51% of the network.
Nodes cannot be attacked to influence the network the same way as miners do. Nodes only enforces the rules locally and does not affect the protocol rules being enforced by others. Even if the other nodes accept a Bitcoin block with a million Bitcoins in block rewards, your node won't, as you're still enforcing the rules yourself, unaffected by what others are doing. As long as your own nodes is enforcing a set of rules, no one else can make your node violate those rules.That being said, Sybil attack doesn't require any proportion of the nodes to be compromised and that is practically the only attack which uses the nodes. You're confused about 51% attack, I'd recommend you to do up more reading on this matter before posting any further.
Thanks for your reply, do you have any recommended reading?