Do people still use terms like “1st disease”, “2nd disease”, etc?
1st disease for measles, 2nd disease for scarlet fever, 3rd disease for rubella, 4th disease for duke’s, 5th disease for slapped cheek, 6th disease for roseola
Do people still use these terms?
I’ve never heard people using them…

These terms refer to the six childhood diseases associated with skin rashes. Not used much anymore except for Fifth disease. These days we just say what it is.