Hi @Noa!
Even if everyone has a copy of both the block and the ledger, one cannot just add a infected/malicious/false ledger on the Blockchain. This is because of the consensus mechanism. For a ledger to be added to the Blockchain, the transaction has to be validated and verified by atleast 51% of the network. Suppose there are a hundred nodes in a network and one node tries to upload a infected ledger, then the other nodes (99% of the network) will reject this transactions.