"At the time these rumors [that "patient zero" was, f'rinstance, an illegal immigrant] began to circulate, public health officials hadn't yet identified a specific source for the outbreak in question; and neither claim about the current measles outbreak's first patient has since been supported by any evidence. All of the individuals who fell ill had visited Disneyland between 15 December and 20 December 2014, but no other epidemiological factors have yet been determined."
Ref: snopes.com
Patient zero has not been identified, . . . and pr'bly won't be. But the present measles outbreak apparently did start in "The Happiest Place on Earth".
n.b. This does not mean it wasn't an unvaccinated illegal immigrant, or a peg-legged midget with a parakeet on his shoulder, or an old geezer from Appalachia on vacation.
For the record poor old DonDiego hasn't been in California for many years.