What happens if a malicious file accidentally makes its way into your backup?

I don't save my files to OneDrive (or any other cloud location) - everything is saved locally. I do use a backup tool though that automatically backs up copies of my local files to various locations in the cloud on a schedule (right now, at every logoff). This is something I've been thinking about for a while, so I thought I'd ask here: what would happen if a malicious file were to accidentally make its way into my backup? Since I use an automatic backup tool, I figured this is always a possibility, assuming Norton or Windows Defender doesn't catch the file first. Would that backup be toxic and therefore unusable? My backup setup doesn't include saving past versions of backups, so once a backup is overwritten, that's it. I'm afraid of one day getting malware and having that malware find its way into my backup, which means I would potentially be unable to recover my files if I had to reinstall Windows.

I have two additional questions related to malware. First (and this might be dumb, so I'm sorry in advance), is a laptop "damaged goods" forever if it gets malware? I've never gotten malware before, but I've read accounts from some individuals who say that you'll never be 100% sure that the computer is 100% clean after you supposedly remove malware. Second, if a malicious file is downloaded on my computer but isn't actually double-clicked on, does it pose a risk to my machine? I'm worried about accidentally uploading a malicious file (probably a PDF) to OneDrive via my iPad (since iOS is known to be hardened against malware, meaning I may not notice), and then having that malware infect my laptop the next time OneDrive syncs on my laptop. Someone told me this risk is very, very small, and having a file simply get downloaded without being viewed or double-clicked on really only poses a risk in the rare scenario that the malware is complex and an exploit in my malware scanner is used when I scan the file. I just wanted to double check here to see what you guys think.

So just to quickly recap:

1. What happens if a malicious file accidentally makes its way into your backup? (And your backup system doesn't include saving previous versions of backups)
2. If your laptop gets malware, is it "damaged goods" forever?
3. How much of a risk is posed if a malicious file is downloaded to your laptop but never actually double-clicked on/opened/run/viewed in a PDF viewer, etc.

Thanks!

Submitted April 10, 2018 at 10:23PM by Longent https://ift.tt/2GQf9hF