I think of asymptomatic infectivity as a sitting war between the virus and the host immune system. The virus has taken hold in a region where it a) does not gro

I think of asymptomatic infectivity as a sitting war between the virus and the host immune system. The virus has taken hold in a region where it a) does not grow at a rate harmful to the host and b) already has access to a spreading mechanism, while the immune system succeeds in keeping the infection away from organs where (either) c) the virus would grow at an unsustainable rate, eventually taking down it's host and/or d) the immune system's counter reaction would be so severe that it would endanger the host. Avoiding c) and d) are in the best interest of both virus and immune system (even though both are obviously not aware of any of that), so both are effectively happy. Sooner or later the immune system will develop a countermeasure nonetheless, but until then the virus will have spread to other hosts generously. Reliably reaching a state like that would be the second best evolutionary outcome of a virus, only topped by permanently resisting the immune system's removal attempts.