So a lethal disease is one that can kill you.. So that means that a lot of diseases can be lethal.. Even the flu! but it depends on a lot of factors like how healthy your immune system is.
Diseases that cannot be treated or cured (like Pierre mentioned Ebola) are particularly nasty because of how they kill you, but diseases like coronary heart disease can be classified as just as lethal because of how many people die from it every year.