I always like to start with the simple answer and then explain. And the simple answer here is yes. It can. Here I’ll describe why and really how risky it can be. And of course some alternative solutions.
A ducted central system typically uses a shared air return. Most commonly in a hallway or open area to most of the house. You’ll find them down low near the ground on a wall or up high in the ceiling. At least here in Orange County and surrounding areas. If your air return (you can identify it by being a large grill, much larger than the ones in each room supplying the air. And maybe there’s a filter behind it also) is low to the ground, odds are your furnace or air handler is a closet directly above or in the garage on the other side of the wall. If the return is in the ceiling, your furnace or air handler is probably in the attic. But what if I have two? My parents two story home in Cypress is setup this way. The furnace is in the garage and the main return is right on the other side of the wall in the living room. And it’s LOUD! There is a second return which was wisely installed (not by us admittedly) upstairs and connects via ductwork. This allows for a more even temperature return as upstairs your home can easily be 5-10 degrees hotter than downstairs.
So with a central return like this, or two. You are effectively pulling in air from a central open location and using that air to distribute into all of the other rooms. So if someone is sick and walks through the living room coughing, it could theoretically get pulled into that return and spread throughout the system and perhaps even into an upstairs bedroom where you think your safe. Without a doubt though, central air systems tend to harbor all sorts of gross things in them. The fans collect dust, mites and all sorts of gunk. Then that gunk gets pushed through a wet evaporator coil and can turn into mildew, mold and other awful things. Of course that evaporator coil doesn’t stop everything and onto the next phase the germs go, the ductwork! And finally right into your room where you sleep at night.
Most central systems are designed for a standard 1” thick filter which are there to simply protect the equipment from large particles of dust and such. You walk down the aisle of your local hardware store and find a $20 magic filter and install it thinking you’ve just improved the air quality in your home. But no, you’ve simply starved your system for air and increased your electric bill and likelihood for a repair call. Instead, consider an air cleaner that can be installed. Such as the Phenomenal Aire R.6 system. Or even a UV light near the musty evaporator coil. A plasma air cleaner such as the Phenomenal Aire system installed by Command Comfort will actually seek out and destroy viruses already inside the home. A UV light, if properly design and installed by Command Comfort should rid and prevent your evaporator coil of mold & mildew.
And if you really want to be safe and energy conscious, consider a ductless mini-split system. Command Comfort is the number one installer of Mitsubishi Electric ductless systems in Orange County. There are no shared returns with these and they are much easier to clean when necessary. Plus they come with advanced filtration built in that won’t restrict airflow. Such as the anti-allergy enzyme filters in the high-wall mount models that come standard. You can clean the air in each room as you keep it at the perfect temperature, and don’t have to worry about your electric bill. Ahhhh, how nice would that be????