In short, no. An active nurture campaign monitors the associated contact list for new entrants. Once the 'added to list' event is captured by the nurture campaign, the contact enters the nurture workflow. Thus if you add a contact to the 'to be nurtured' list while the campaign is inactive, the contact will NOT be nurtured.
As well if you set up a nurture campaign, associate a list full of contacts, then enable the campaign, again, the contacts will NOT enter the workflow. Thus enabling/disabling a nurture campaign will not add the list members to the nurture workflow. Only those contacts that are added to the associated nurture contact list while the nurture campaign is active will enter the workflow.
Contacts that are already being processed in a workflow will not leave a wait state if the campaign is deactivated. Re-activating the campaign will allow the contacts to continue being processed.