Fungus in the form of mold is everywhere. It's spores irritate our respiratory tracts the same way plant pollen does...by trying to mate with our mucose membranes!
For most people this is a minor annoyance but for those who have asthma or are immune compromised, opportunistic infections can be caused by certain molds. Tests now exist for immediate confirmation of the existence of some of these molds in your home. I am certified to do these tests if warranted.
The main weapon against mold is to deprive it of moisture so remmediation will mainly involve discovering and abating sources of moisture intrusion into your home. Even dead molds can cause allergic reactions so molds must be scrubbed free of all surfaces and if this is not possible, drywall, insulation, wood framing and other porous building materials will have to be removed.
Click on image for EPA info on mold.