Well at least we agree that anarcho-capitalists don't have realistic ideas. This might shock you, given my presumptions about your presumptions about me, but I'm even in favor of forcing people to wear masks or stay indoors if they are transmitting a deadly disease. You can't come to that idea from anarcho-capitalism. But you CAN come to it from the ground up, you don't have to reach it from the top down.
But I'm not trying to reach it from the top down. I don't know what you're insisting on a directionality. Individual and collective interests can be in tension with each other, and the maxima of the latter seems to be highly contingent on some concept of the former. As demonstrated by collective action problems (see: tragedy of the commons, uncaptured negative exterbalities, incentive structures for non-violent dispute resolutions like rule-of-law).