First we need to understand what mold or fungi is. Mold is an organism that will digest cellulose and organic materials when water or elevated moisture conditions exist.
Part of the fungi is directly rooted into the wood or cellulose-based material, given enough time the fungi will slowly digest or rot the wood or cellulose base material.
The part of the fungi that is above the surface of the material is the fruiting body. The fruiting body is the part we typically can see as a mold colony.
At the end of the fruiting body is where the mold spores grow.
These spores are released into the air. These airborne spores will stay airborne searching for new wet or damp wood and cellulose materials to colonize.
This is the most common way that mold spreads throughout a home.
Mold spores floating through the air are constantly landing on surfaces every minute of every day looking for wet surfaces or humid conditions to give them enough moisture the root and grow into a new mold colony.
This cycle of a mold spore landing rooting, growing a fruiting body, then creating more mold
can happen very quickly when wet or humid conditions exist.
There will almost always be a minimal amount of mold spores in almost every environment on the planet.
Having a minimal amount of mold spores present is typically not a problem.
When materials become wet and humidity is above 60% conditions to promote mold spreading exist.
To prevent mold growth wet materials need to be dried within three days or less.
Mold can and will spread quickly.
I have seen mold grow several feet in a few weeks between the time a project was estimated till the remediation work was performed.
Typically having wet materials combined with elevated humidity and warm or hot temperatures encourage fast spreading mold. When a water heater malfunctions and goes unnoticed the hot water, steam, and elevated humidity encourages mold-growth sometimes in just a matter of a few days. The resulting mold growth can create an extensive remediation project.
In other areas, such as a crawlspace where the humidity may only be elevated in the summertime, mold will grow slowly just a few inches or a few feet every summer. But given several years the result is a moldy crawl space that can affect the air-quality within the living spaces of the home.
Mold may also spread by the roots growing into, across, and through the material it is digesting.
When you cut a mold damaged wood joist you can see that the mold roots have grown and digested in every direction throughout the wood. Mold spreading through its roots is a slower but more damaging process.
Most often mold spreads by mold spores floating through the air and landing on materials that are wet or located in humid conditions.
Every time a colony creates more mold spores and begins to spread the mold spore count within the space becomes elevated. When mold spores become elevated it creates unhealthy air and unhealthy living conditions.