Final answer:
The overgrowth of an unaffected species typically leads to a decrease in most normal flora, disturbing the microbial balance and potentially causing health issues.
Step-by-step explanation:
The effect of overgrowth of an unaffected species on most normal flora is generally a decrease in the most normal flora. When one microbial species grows excessively, it can disrupt the balance of the normal microbial community. This is often due to the overgrown species utilizing more resources or creating conditions that are unfavorable for other species. For example, when an unaffected species like the antibiotic-resistant bacterium Clostridium difficile flourishes after antibiotic treatment, the normal gut bacteria population is decimated, leading to severe imbalances and health issues like chronic diarrhea. Therefore, the overgrowth of an unaffected species can decrease the diversity and abundance of most normal flora, creating potential health problems for the host.