Can a Virus Enter via Ubuntu and affect Windows in dual boot?

"It's certainly possible to infect Windows through Ubuntu. It can happen if you have an infected file that gets transferred from one OS to the other; however, an infected file that lives only on the Linux partition will probably not have any affect on the Windows OS.

The easiest way to keep Windows clean is to use an antivirus application in Windows. It might be a bit overkill to also install an antivirus app in Ubuntu as well; however, if you want, you could use something like ClamAV to scan files before transferring them to Windows. There's more info on ClamAV in this question on SuperUser.

The good thing about ClamAV is that it's an on-demand scanner, so it doesn't consume any resources when you don't need it. That way, when you transfer files from Ubuntu to Windows, all you need to do is scan them before copying them over to the other OS."

"It's certainly possible, although far-fetched:
If the Linux malware has read-write access to the Windows partition, it can then modify any Windows file that it likes, and that way infect the other partition.

The malware needs to be designed as cross-system, but with the sophisticated multi-vector attacks that we're seeing today, everything is possible.

The only protection would be to ensure that Ubuntu doesn't have write access to the Windows partition."