If there hasn't been any changes to your /hosts file then it may have been a false-positive. The image you provided doesn't really provide any additional details.
You can exclude the hosts file from your scan:
This issue occurs because Windows Defender may determine incorrectly that the Hosts file was changed by malware, such as adware or spyware. Typically, malware programs change the Hosts file to redirect users to malicious websites. Therefore, Windows Defender may detect the Hosts file as a security threat.
Hosts file is detected as malware in Windows Defender
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