Very hard to say... theoretically you should be able to use your ledger on an infected pc and still not lose your funds...

I can think off a couple very remote scenario's that happened, or were at least discussed in the past:

• You saved your seed in the cloud, and your account got hacked (icloud, dropbox, gmail,...)
• You saved your seed on a physical carrier, and somebody found said carrier
• You entered your seed in a different (vulnerable) wallet
• You were the victim of a clipboard virus (eventough, this would have required you to actually make a transaction)
• You received a device that had been tampered with... either preloaded with a seed, or plain fake
• You were the victim of an evil maid attack (somebody had physical access to your device)