Missionary lost due to Attrition?

Sent a missionary to spread the word in Assyria.

He makes it to the capital. Spreads the gospel. Then I hear the you lost a unit sound. I click on it and it is my missionary at the capital. It states he was lost due to attrition.

Did they mean murder?

And can I do the same?

I am in the early stages of a my 1st marathon match.

Whenever a missionary ends its turn in a civ that doesn't have open borders, it loses part of its strength. When its strength is fully depleted, it dies.