why does A/C "funk develop on late-model cars?

Currently dealing with a stinky A/C system on an Infiniti Q50. A google search showed a couple of discussion forums where many Q50 owners appear to be dealing with the same problem.

Then I googled other makes/models, and I’m seeing the same discussions. IOW, it seems that stinky A/C syndrome strikes indiscriminately.

Late-model cars seem to be commonly fitted with a cabin air filter (as is mine). Presumably this filters out the majority of airborne particulates that could conceivably provide a food source for whatever bastard organisms take up residence on the evaporator. So why is this still such a common thing?

It’s commonly claimed that regular use of the “recirculation” mode makes your car more likely to develop stinky A/C syndrome. Why would this be? Is the recirculated cabin air not passed through the cabin air filter?

not being familiar with the design of that particular car, I’d WAG that its problems stem from an HVAC manifold which allows water and other contaminants (twigs, bits of leaves, pollen, etc.) collect on or near the evaporator or filter. a pile of organic matter on the filter will stink, and the filter can’t filter out stink.

The problems described above can be rectified with relative ease.

Not being familiar with that particular model, there are some standards. Near the windshield exterior by the wiper blade arms is a little vent, you can spray that with disinfectant spray (that destroys mold and mildew) as well as open up one of the small ducts beneath the dash above the accelerator pedal and brake and spray it in there, spray inside the regular HVAC vents on the dash and under the seats, then take out the blower motor and do the same (after removing the bits of detritus that have collected and cleaning it down). Turn on your A/C on its recirculate mode and keep spraying these parts while it runs for a few seconds, then replace your cabin air filter if applicable. Keep the windows open and let it run for a few. The smell will be eliminated.

EDIT: I forgot find the small drainage tube underneath your car where condensation is supposed to leak and make sure it is not blocked with road debris. That can contribute to moisture buildup in the system and will help bacteria thrive.

Don’t know about this claim but I do know when the fan is on recirculate, the filter is bypassing.

Just a guess but…
Maybe a prolonged period of full on heat would kill off the funk.
Park the car in full sunlight on a hot day. Turn temp to max and blower to full.

Remove all the stuff that might melt first. But can’t guarantee actual interior plastic car parts won’t melt.

If you can find the internal vent that draws in air when in recirculate mode. You can try spraying some mold killer stuff in that vent while it is drawing. Just don’t stay in there breathing the stuff in for too long.