Measles makes your immune system forget. It's called immunological amnesia: it literally wipes the slate clean, and you are now vulnerable to any disease you already acquired immunity to. https://twitter.com/pookleblinky/status/1333582339960672262

Until the measles vaccine, everyone's immune system would get periodically reset.

People who'd survived, say, mumps, would get it again a decade later.

The measles vaccine is a triumph because, for the first time in history, it made immunity permanent.

You didn't have to worry much about catching a disease again you'd gotten as a kid.

The measles vaccine made every other disease less lethal.

You wouldn't get 2 rounds of rubella, the first leaving you deaf in one ear and then the second giving you rheumatoid arthritis or kidney damage.

With the measles vaccine, you could actually *kill* other diseases.

Until then, it was only a matter of time before a measles outbreak reset enough people's immunity to allow those diseases to return.

How long ago?

The first measles vaccine was made in 1963. In 1971, it was enhanced into the MMR vaccine to protect against the synergistic diseases mumps and rubella.

Actual permanent immunity is only 57 years old.

Permanent immunity is older than woodstock.

Up until then, no vaccine, or herd immunity, could promise permanent immunity.

A measles outbreak a decade later could bring that disease back with a vengeance after wiping out any immunity acquired.

In 1963, humans could finally become permanently immune to diseases, not merely temporarily until the next measles outbreak wiped everyone's slate clean.

If you're younger than 57 years, you lived in a world where diseases could be killed. Where outbreaks of every kind were expected to end. Where diseases could not gain a foothold and erupt every few years like clockwork.

The measles vaccine made every other disease less dangerous.

When it fell, it brought down every other disease.