If you do want to run all Nessus scans, including the dangerous ones, you can select that option. You can also choose to use a pre-defined policy you have already customized using Manage Policies.
Vulnerability Scanning With Nessus
From Tony Bradley, CISSP-ISSAP, for About.com
Nessus scans by default using all scans and plugins except for the scans that are deemed potentially "dangerous". Dangerous plugins may potentially crash target systems and should only be used if you are sure there will be no impact to a production environment.
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