How common is it for Astartes to turn to Chaos in the current timeline?

Obviously in the Horus Heresy we had entire companies turn to Chaos, but how often do individual or small numbers of loyalist space marines fall in the current timeline?

Is it something that just never happens?

Or is it something that some Chapters must be ever vigilant for, like how Blood Angels always have to worry about the Black Rage.

I'm interested in examples after seeing some cool themed conversions of fallen Ultramarines etc.

Bonus question, any instances of Chaos Primaris?