Your content is safe, even if content from another user is infected. Campaign doesn't run any code or open any attachments that users can upload to the servers. The servers are immune to infecting other user's content if one of these files is infected.
By default, Campaign rejects scripting in emails so the creator of a message would have to deliberately insert a virus into the body for the message to contain a virus.
Campaign does not scan message content. Responsibility for creating messages that are free of virus-infected attachments, links to virus-propagating Web pages, or virus-enabled email messages lies with the creator of the message.