I did not knew the answer hence I checked on the Wikipedia, from same I found: “Mold is found everywhere and can grow on almost any substance when moisture is present. They reproduce by spores, which are carried by air currents. When spores land on a moist surface suitable for life, they begin to grow. Mold is normally found indoors at levels which do not affect most healthy individuals.”
And I guess depending on the weather their concentration and growth can vary, and further they can lead to few minor health problems, such as cold, respiratory issues when the weather changes.