Hi @Hadi Ahn!
These are pretty interesting questions you have, so I'll try to help you out as much as possible.
1. The NAS has its own app called Anti-Virus Essentials, which you can download and install for free and will allow you to protect your data in such situations. Here's a KB article with more information about the app and a guide on how to install it too:
Once the device finds any viruses, you can choose either to delete or quarantine the file. Also, you can choose to ignore it as well if you want. You can take a look at this KB article about how to exactly do that:
2. As long as you're the admin you can add or change the users password of any share and by doing so - you can access the information on their share. For more information you can take a look at the manual of the specific device, for instance this is the one of WD My Cloud Mirror, more specifically on page 22:
3. Yes, there are few backup programs you can download and use for free from our website, like WD Backup, Acronis True Image WD Edition for example.
Hope this helps a bit and feel free to ask any questions you may have.