Seriousness isnt on the basis of how many it has killed.
I mean ffs you cant cure cancer buy you can small pox. Also these diseases killed back when medical science didnt have a clue about anything and the world most people lived in was breeding grounds for it to grow.
none of that applies today, medical science knows about these things and the world is not a breeding ground for the plague or anyhting like that.
AND FINALLY, people did get cancer in the past, they just didn't know what it was. Anyone can get it so it is naive to think only the old were susceptible to it over the last 2 centuries. Clearly people didnt know every illness going back then.