In Windows 10, you can turn off Microsoft Defender Antivirus real-time protection, but it will only be temporary unless disabled. Some malware can also be programmed to run at unexpected times, not only when it's installed.

Malware can infect your PC without your knowledge: it might install itself from an email message, when you connect to the Internet, or when you install certain apps using a USB flash drive, CD, DVD, or other removable media. Microsoft Defender Antivirus helps protect your PC against malware (malicious software) like viruses, spyware, and other potentially unwanted software. This includes updates related to keeping Microsoft Defender Antivirus up to date. Dedicated protection and product updates.This includes near-instant detection and blocking of new and emerging threats. It also includes detecting and blocking apps that are deemed unsafe, but may not be detected as malware. This includes always-on scanning using file and process behavior monitoring and other heuristics (also known as 'real-time protection').

Behavior-based, heuristic, and real-time antivirus protection.

Next-generation protection services include the following: Next-generation protection brings together machine learning, big-data analysis, in-depth threat resistance research, and the Microsoft cloud infrastructure to protect devices in your enterprise organization. Microsoft Defender Antivirus is the next-generation protection component of Microsoft Defender Advanced Threat Protection (Microsoft Defender ATP). In addition to this real-time protection, updates are downloaded automatically to help keep your device safe and protect it from threats. Windows Security continually scans for malware (malicious software), viruses, and security threats.

Your device will be actively protected from the moment you start Windows 10. Windows 10 includes Windows Security, which provides the latest antivirus protection. Starting with Windows 10 version 2004, Windows Defender Antivirus as been renamed to Microsoft Defender Antivirus. How to Enable or Disable Real-time Protection for Microsoft Defender Antivirus in Windows 10

Disabling windows defender