English.

They thought it was a language. It looked and acted like a language. Of course, it could be used to ‘communicate’… like other languages.

But after it destroyed their minds and laid waste to their relational intelligence and their remaining human cultures almost entirely — their grandchildren realized its actual function: it was a highly aggressive cognitive parasite. The cognitive analog of HIV. It went after the intelligence necessary to detect its identity and reject it — first. Once those obstacles were converted to assets, it went after the rest of the world.

It just so happened that certain individuals were slightly immune. Just enough to survive the initial onslaught, and, from then on, to act in the predator’s interest. So it looked just like any other language…

…until it started to spread.