Isolating affected systems from the network will prevent the malware from spreading to other systems or exfiltrating data. Network isolation could be physically disconnecting the machine or logically separating it using network controls. Updating antivirus definitions might not be effective if the outbreak is due to new or undetected malware. Shutting down non-essential services doesn't directly prevent malware communication, and installing security patches, while an important practice, doesn't immediately contain an active outbreak.