Few furnace filters, except expensive HEPA filters, do much about trapping or killing germs. Electronic air filters will reduce somewhat the air born germ count because the high electrical voltages do kill some of the bacteria passing through them and probably all that is caught on the plates. UV light kills airborne germs even more efficiently and now you can buy DIY kits for installing a UV lamp inside your furnace duct. Probably the greatest need for such a device is when you have air conditioning coils in the furnace, with their constant presence of moisture providing a great breeding place for germs.
The one I installed on the TV show is called Phase 2 -- referring to your furnace filter as the first phase and then the UV lamp as the second phase of cleaning the air in your house. It is placed inside the duct after the furnace, and hence after all the filters, catching the air just before it heads into the distribution ducts.
The zone of air about 3 or 4 inches around the lamp will get sterilized as the air passes by. You must be careful to never look at the lamp without special UV filtered glasses while it is lit, hence it has a special filtered peep-hole if you want to look in and see if it is working. You do have to remove it occasionally to wipe it clean. If it is covered in dust, the UV light can't get out to do its job. It won't kill all the germs that pass by but it does reduce the germ count significantly, and if you leave the bulb on and the furnace fan running, as is often recommended for other reasons, it will cumulatively sterilize the air in your house, reducing the passing around of things like colds.
This particular kit is about one of the best DIY kits I have ever seen, distributed by a company called Field Controls. They supply a self adhesive template that glues to the ductwork to guide the cutting of the holes. The template remains after the job is finished and provides the instructions for replacing bulbs. They provide all the drills necessary and the unit just plugs into an outlet to avoid having to take on any wiring, and a video to explain about the device, its installation and maintenance.