is a disease common to both World 12A
and World 12B
, and probably others. Despite the wishes of those who catch the disease and those who have to look after them, magic has almost no effect on it.
Julia, Roger and Janet all catch measles (in that order) in "Stealer of Souls"; Cat and Tonino are saved by having already had them.
(The people of the Moor are also familiar with measles; Gerald Masterfield in Power of Three has just had them, to the surprise of Gair and his siblings, who didn't know Giants could get measles too.)