(Not a Teacher)
It depends on the context. For example:
"Heart disease is the number one cause of death in the US."
"Atherosclerosis is a heart disease."
"Skin disease afflicts millions worldwide."
"Acne is a common skin disease."
Also note that you will hear 'cardiovascular disease' fairly frequently as well. This is the more scholarly term for this class of disease. As for skin disease, I think there is 'dermatological disease', but I don't think that's particularly common outside of certain circles.
Retired English Teacher