Also called Subacute sclerosing panencephalitis (SSPE), it affects like 1 in 5,000 people who contract measles (vaccinated), but jumps to 1 in 609 in the unvaccinated.
Basically, you get measles and then seem fine, but anywhere from months to 15+ years later, you develop brain inflammation, seizures, spasms, blindness, and coma, and it’s basically 100% fatal. The disease attacks your nerves and brain. There’s no treatment or cure, and it hurts the whole time you’re dying. It can take months or more of excruciating suffering to kill you. It’s similar to rabies, in that you lose all control and are guaranteed a protracted, painful death.
It’s preventable by getting the measles vaccine.
Many diseases are organisms that want to procreate. So it makes sense to let you get on with life for a while, so you can infect others (thus procreate themselves).
If they kill you too fast, they die, too.
Once you’ve spread them, it makes little difference how horribly they kill you.