:salamext:
On my opinion, i think you should look into the problem thoroughly because it may be a form of regret later, and both people could end up getting hurt more than if one person was to say no to the proposal.
To define hepatitis C; it says on the following link:
An infection of the liver that is caused by an RNA virus, is transmitted primarily by blood and blood products, as in blood transfusions or intravenous drug use, and
sometimes through sexual contact. Most cases of non-A, non-B hepatitis are of this type.
source:
http://www.answers.com/hepatitis+c&r=67#after_ad1
So you may want to look deep into this issue and do alot of research on the illness, because if you were to be affected with it too - then it may affect your life permanently too. And naturally, man would hate the idea that they got it off someone else, while trying to do a good act - even if it wasn't really the other persons fault.
This is why, having alot of imaan and taqwa is important - because shaytan will try to do anything to cause arguments and breakups between a husband and wife.
Also - we don't know the ruling, hence its important that one does research on that too, thoroughly insha'Allaah. Even if it means asking your question a scholar who is qualified in the sciences of medicine, or by asking some qualified scholars online.
Also, no matter what anyone was to say - please remember that istikhaarah is really important before the proposal is accepted/rejected etc.
Allaah Almighty knows best.
:wasalamex