Final answer:
Long-term antibiotic use commonly results in new infections by destroying the body's normal flora, allowing overgrowth of resistant pathogens and contributing to antibiotic resistance.
Step-by-step explanation:
The most common reason that long-term antibiotic therapy can cause new infections is because it destroys the body's normal flora. Antibiotics are designed to kill bacteria causing illnesses, but they also eliminate the good bacteria that protect the body from infection. The loss of normal gut bacteria can lead to an overgrowth of antibiotic-resistant species, such as Clostridium difficile, resulting in severe gastric problems like chronic diarrhea. This overgrowth is known as a superinfection, which occurs when the protective microbiota are decimated, allowing a pathogen resistant to the antibiotics to proliferate and cause a secondary infection. Inappropriate use of antibiotics facilitates the development of antibiotic resistance, meaning that the bacteria evolve and become immune to the effects of antibiotics, making treatments less effective. This creates a significant health concern as resistant infections can lead to more severe illnesses and complications.