Some things get started by others starting Like finger-snapping at a beatnik party

This seems like a safe generalisation: The greater the gullibility of Someone on the Interweb, and the more difficulty they have separating their desiderata from actuality, the more likely they are to self-describe as unblinking skeptics, doing Their Own Research unswayed by authority. Case in point: this cockwomble regurging one aspect of the collective antivaccine memory:

In the 1950's, mothers used to hold measles parties, so that all the children could get it over with

Can you imagine that. Mothers would take their healthy children, and deliberately expose them to the most infectious disease in the world

None of those mothers were ever jailed

— Sheldon Walker (@Sheldon_Walker_) December 9, 2019

Oddly, these 1950s festive events so fresh in the memories of so many Twiddle nyms have left no contemporary documentary traces. Yet if they had occurred, there would be citations aplenty: we know this from Goofle searches for Rubella parties (German measles). These were a thing in the 50s & 60s, after a high-profile doctor promoted them as a way of ensuring young Anglo-saxon girls were ready for their roles as fecund broodmares.

Other meducators, less eugenics-conscious, were less enthusiastic.

Those vivid though anachronistic recollections of 1950s Measles Parties must have come from an alternative time-line (in accordance with the 'Garden of Merging Paths' model of quondam fairy). Perhaps they are another manifestation of the Mandela Effect. Fifty years later they were retconned into the Antivax shared narrative, to the extent that a Health Freedom Fighter writing in 2011 could solemnly assure US readers that Britain and continental Europe are bastions of liberated medical self-reliance where pox-parties are commonplace even now.

Here's another version from 2010. Someone assembled scraps of unreflected, half-digested other people's writing into a book-shaped Reference Guide to Emerging Biological Threats.

The retconning began accidentally in 2001, with an brashly confident, ill-informed Guardian life-style column (but I repeat myself). I can only suppose that the columnist, staring vacantly into space, allowed a fleeting half-inchoate semi-recollection of the Rubella Party phenomenon to escape from her brain and make an unsupervised dash down through fingers and keyboard onto the page. From there it was quickly codified as an article of faith.

Popular in the 1950s before mass immunisation, the measles party is making a comeback. 

There is a scholarly treatise waiting to be written about collective False Memories, exemplified by the inception of a Guardian columnist's fabulation into the communal antivax memory. But that sounds like work. I would rather wait for enough people to blog about the scholarly paper I wrote, and to remember reading it, until it becomes accepted as part of the consensus past and I can claim it on my CV.

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Turns out that Vincent Ianelli MD at Vaxopedia had Done His Own Research and come to identical Rubella-related conclusions, though a few months earlier.

I did unearth a 1956 Good Housekeeping reference to a post-measles party, with invitations extended to girls who had caught the disease and were convalescing, as a consolation.

Some sources in the 1930s and 40s reckoned that "measles parties" had existed in earlier generations, back in the dark ages of medical ignorance, as a misguided superstition.