Hepatitis C
Transmitted usually by needles either accidentally (drug users) or in a medical facility.
This is a debilitiating and fatal disease for which there is no treatment or vaccine so it is best as a traveller not to put yourself at risk. You do this by having the least medical care overseas as possible, so go with your Rabies shots, tetanus already boosted and your malaria pills and do not front up for preventable medical care in developing nations!
You may come home with AIDS, Hep C or some unknown emerging infection (just think about how AIDS started...) if you take medical care for granted overseas!