Avast says Protected MacBook file infected with adware

You fix it by entirely removing Avast per the vendor's instructions. Like all other anti-virus software on the Mac, they simply cannot find viruses, and instead produce false positives for files located in the operating system, when they are not simply interfering with the built-in security already present in the operating system.

What you can and will find occasionally in the operating system is the presence of ad/malware that tagged along on some file that you downloaded, and then gave administrator permission to install. Anti-virus software will never stop you from doing that. For ad/malware, we (your fellow users) recommend the free Malwarebytes for Mac product which is purpose-built to detect and remove this form of infestation. It is not anti-virus software.

Add-on anti-malware is a trash-fire of false positives and of false negatives. And of weird problems.

In the particular case of Avast, of uploading and reselling all your web activity and your purchase history, too.

Remove Avast.

As for contending with malware on Mac: Effective defenses against malware and other threats - Apple Community

scanned my MacBook with avast and it says the file /private/car/db/dyld/Dyld_shared_cache_x86_64h is infected with MacOS:Pirtit-BU (Adw)

This is, of course, utter nonsense.

/private/var/db/dyld/Dyld_shared_cache_x86_64h is maintained by macOS. It exists to minimize the time it takes to boot your Mac, and should not be touched. It is kept updated normally by Software Update.

If on the other hand something is installed at /private/car/ then further investigation is needed. A folder by that name has no reason to exist.

Get rid of "Avast". Rule 1 of Macs is don't install junk.

Uninstall AVAST.

Char172 wrote:

So I just ignore it?

No, uninstall AVAST.