This is of no educational value whatsoever, but I couldn’t resist adding it.
In the onrush of facts to be memorised to get through medical school, I spared very little time for wondering why slapped cheek is known as ‘fifth disease’. However, it is a good question, and a doctor writing in the BMJ has now answered it. Apparently, first disease is measles; second disease is scarlet fever; third disease is rubella; and fourth disease seems to have been a medical misclassification and has never been definitely clarified. Also, sixth disease is roseola, and there may or may not be a seventh with the non-snappy name of acute febrile infantile mucocutaneous lymph node syndrome, or MCLS for shorter.
What I’m not yet totally clear on is what the criteria actually are for making the ordinal list, given that plenty of other diseases have been diagnosed during the thousand-odd years in which this list was being compiled (measles was apparently first scientifically described in the tenth century). Infectious disease with a generalised rash, I’m deducing, but it would be interesting to know more about that.