so, serious from this point of view is an illness that prevents you to function in society. for example, it is so severe that you cannot work or go to school.
let's say someone has cancer. it is serious because the illness keeps the person from working and has a toll on the medical system and social services.
something like diabetes, thyroid malfunctions....not so serious because you can live with these and be a productive member of society.
Diabetes is actually a very serious and expensive disease and leads to many people being disabled.