Exanthems

I did not know this:

In antiquity, [exanthem - "eruptive" or rash-involving] illnesses were all lumped together. Eventually, a distinction was made between measles and pox (with growing clarity over about a millennium). However, what people called the “pox” and what was then called the “measles” are now known to each include a variety of distinct diseases.

Six separate childhood exanthems were defined from what was once called the “measles.” In the early part of the 20th century, these were often referred to by number. Measles and scarlet fever were the first two to be separated. Rubella (German measles) was called “third disease”; atypical scarlet fever was “fourth disease”; erythema infectiosum was (and is) “fifth disease,” and roseola was “sixth disease.”

Chickenpox and smallpox, the other two classic childhood exanthems, were recognized as separate from each other in the 18th century. These both had blisters, or pox, that set them apart from the red rashes of the other group.Today, dozens of exanthems are recognized, including adenovirus, anthrax, mononucleosis, Colorado tick fever, mumps, cat-scratch fever, rat-bite fever, Rocky Mountain spotted fever, relapsing fever, meningococcemia, typhus, hand-foot-mouth disease, and many others.

Exanthems (DrGreene.com)