A few reasons your carpet may not be salvageable:
Carpet on the stairs is generally not removed for safety reasons. Exposed tack strips or staples, especially on steps, is dangerous. However, there are situations where removing carpet from stairs is necessary to prevent damage to hardwood steps or in the case of sewage contamination. Removing tack strips is not advised, as this may damage the hardwood or the subfloor. Extreme caution must be used whenever the tack strip is exposed or when the carpeted stairs are damp. If carpet left on the stairs is unsalvageable, it will be removed when new carpet is installed.
Sometimes you will hear terms like “toxic mold” and “black mold” used to refer to molds. While identifying the type of fungus or mold may be interesting, it doesn’t affect the course of action. If mold is present, the CDC has strongly recommended that it be removed, no matter the type*.
*Source: “Facts about Mold and Dampness.” Centers for Disease Control and Prevention. 18 September 2012. Web. http://www.cdc.gov/mold/dampness_facts.htm