"Diabetes is a heavy disease."
Translation:Diabetul este o boală grea.
Serious might be a better translation for grea than heavy when referring to a disease.
You could say grav, which means it is grave/serious. (Or grava in this case, sorry for the missing accent).
Hello Duolingo, you just cannot say "heavy disease" in English, it makes no sense. You can say "hard", "tough" or "difficult". "Heavy" is normally used just with weight, apart from a few idioms like "with a heavy heart" meaning 'sadly and reluctantly'. (Native speaker)