Hi denvergirl,
Here's the thing. Your diabetes would probably disqualify you as a primary applicant, but wouldn't as the spouse of an applicant or if you were sponsored by a permanent resident or citizen of Canada as a family member (if you got married, for instance).
The reason for this is they try to determine whether you will put "excessive demand" on public health services and therein deprive other Canadians of equal access to treatment. But the excessive demand requirement is waived if you aren't the primary applicant, or if you're sponsored as a family member.
Here's a link:
http://www.canadavisa.com/immigration-medical-inadmissibility.html