The CBIC Certified Infection Control Exam Study Guide (6th edition) explains that the effectiveness of disinfection depends on multiple physical, chemical, and biologic factors, but virulence of the organism is not one of them. Virulence refers to an organism’s ability to cause disease in a host, which is a clinical characteristic, not a determinant of susceptibility to disinfectants.
Disinfection efficacy is influenced by factors such as the type and number of microorganisms, particularly their intrinsic resistance (for example, spores are more resistant than vegetative bacteria), making option C a true determinant. The amount of organic material present (option B) is also critical, as organic matter can inactivate disinfectants or shield microorganisms from exposure. Likewise, the length of exposure (contact time) to the chemical agent (option D) is essential to achieving the desired level of microbial kill and is specified in manufacturer instructions for use.
Virulence does not affect how easily an organism is destroyed by a disinfectant. For example, a highly virulent organism may be easily killed by a low-level disinfectant, while a less virulent organism such as a bacterial spore may be highly resistant. Therefore, virulence plays no role in determining disinfection effectiveness.
For CIC® exam preparation, it is important to distinguish between clinical severity and microbial resistance. Disinfection effectiveness is based on resistance characteristics and process variables—not on how dangerous the organism is to humans.
