Often, heating and cooling systems will mask that there is a humidity problem, and it's not until you actually test the room with a humidity sensor or plug in a dehumidifier that the condition is obvious.
While it's commonly believed that black mold is the most dangerous type of mold to have, not all species of black mold are toxic. Such variety makes it difficult for homeowners to tell what type of mold they're dealing with, and whether or not it's toxic. The only way to tell for sure is to send a sample to a laboratory for testing.
When mold spores find a hospitable area with sufficient moisture to grow they take hold and spread. They love surfaces of cellulose: paper and wood products being the most affected usually. In homes with infestations you will often see the black toxic mold growing on the drywall or wall paper.
One of the problems that comes with living in an area where the air is humid at certain times of year, is toxic household mold. This seemingly innocuous issue can cause not only serious damage to property, but more importantly, it can be extremely hazardous to residents' health.