You hear of them all the time, seeing them screaming on the interwebs, doing podcasts, videos, protesting when new vaccines show up. When the immunoflu was invented, they screamed even louder. That is when the "pure lifers" started. The ones who kept themselves in the equivalent of "bubbles" to avoid them, and their children, ever catching the immunoflu. A large number of them went to a planet to settle in and live. The planet was, if not for the terraformed domes, uninhabitable. But inside the domed cities, it was very pleasant. Until that one visitor. They had kept themselves isolated for four generations, then that one visitor came, a man from the Alliance. They'd never let anyone into the dome before without the person in question, even tourists, being forced to wear hazmat suits to make sure that nothing the person carrying ever got in, but accidents always happen, eventually.
He was in a crowded mall, everyone knew it was an accident. But that didn't help how angry the people were. He was sitting, reading a map, he'd been lost. Several teenagers were roughhousing when one was shoved into him and he crashed backward into a crystalized statue. The hazmat suit already had some unseen potential flaws, was badly damaged. The immunoflu he carried got out into the community. This was a "Pure Life" society.. and now they were having to deal with being immunized whether they wanted to be or not.
(notation - In a nutshell a society that eschews all vaccinations even ones from the immunoflu, like real life modern day antivaxxers, build their own society. They're cut off from the biome of the rest of the galaxy, and then.. there's an accident.) -- DaniandShali
Disasters don't have to be instant. The worst of them happen in slow motion. To the tune of For Want of a Nail, a nation, a polity, an identity, or a culture can die. Dependant on the culture, this can be beneficial to those who formerly held it dear.
For want of immunity, lives were lost... Once upon a time, there was disease. Smallpox, measles, mumps... diseases that could maim. Diseases that could kill. Some people hit on the idea of deliberately infecting children with a weaker disease so that there wouldn't catch the more menacing one. They named it after the animal that the weaker disease came from. Vaca meaning cow, for the cowpox that forestalled the attacks of smallpox.
One would expect that such a breakthrough would be welcomed with open arms and cheering in the streets. No such luck. There was widespread panic, misunderstanding, and fear of the new process. Over the intervening years, very little changed. For every new intervention, there was an equally opposing reaction. Confusion and fear abounded. Someone, somewhere, drew a correlation between these vaccines and the rise in diagnoses of autism. Many people feared a mental disorder more than they feared the maiming and killing diseases that the vaccines prevented.