What disease that can kill you when your eighteen?

Anything. Pretty much any disease. The most common cause of death in this age group is accidents, though. Infectious diseases, including HIV are fairly prevalent. Cancer has "gotten younger" as well as some cardiovascular diseases. Drug and alcohol abuse are high on the list if not second.
Anything. Living a healthy life will avoid any serious illness except the inevitable.At the age of eighteen, you are vulnerable to drugs, alcohol, lack of sleep, abuse of anything that will lead to sickness because of influence of friends. You still need parental guidance. Good luck !