Windows Defender by default doesn’t scan any external removable drives attached to the system or any of the email attachments downloaded. Without any standard antivirus application, this can pose a security threat if left unchecked.
In order to include both of these too for scanning :
1. Open Control Panel > Windows Defender
2. Select Tools > Options.
3. Select Advanced tab in column and enable the check mark next to “Scan removable drives” as well as “Scan e-mail”.
All done.