[2020] What raises the Max Health Cap with ISS?

I've looked around but everything I see is either related to the old Wellness System, or about refilling a bar after damage using bandages - which I've figured out.
However, at the start of the game you have 100 Health Cap. In my current playthrough, where I suck eggs (literally sometimes), my Health Cap is at ~127. Clearly at some point it raised.
However, I don't understand what causes that raise. Putting perk points in Strength and Fortitude didn't seem to make the caps change, and the tooltips don't mention it where you'd think they would.

I am not wounded, either in the temporary sense of just now getting whacked by a zombie or in the long-term sense of taking too much damage and having the cap lowered - I just want it higher. What makes the cap *go up* beyond what it was before?