This is one of the nastier issues you can encounter in your home or apartment. If you are in an apartment, the landlord should be notified immediately as mold spores in the air can cause health issues for you and your family.
Mold spores are actually always in the air, but to a degree that is safe and harmless to humans. But if it shows up in ductwork, attics, walls, or under the house it can cause all sorts of issues. Sometimes is leaves a musty smell, but other times it is virtually undetectable.
There are a lot of different kinds of mold, each with its own characteristics. If you want to learn a bit more about the types, check out the Richmond Mold Remediation Pros site for details about all of them. This site will walk you through all the types, where they can occur, what issues they can cause, and explains how a professional would get rid of it, including an explanation of the different types of equipment required to do the job.