Final answer:
Bacteroides fragilis is normally harmless within the gut but can cause infections if it enters the bloodstream. Escherichia coli is generally non-pathogenic, but some strains can lead to gastroenteritis. Plasmodium falciparum, unlike the previously mentioned microbes, causes malaria and is a protozoan.
Step-by-step explanation:
Bacteroides fragilis is a significant component of the gut microbiota, playing a crucial role in maintaining the health of the colon. While typically harmless, if B. fragilis crosses into the bloodstream due to injury or surgery, it can lead to infections. Another gut resident, Escherichia coli, primarily exists as a benign part of the intestinal flora. Certain strains, however, equipped with virulence factors such as toxins, can cause gastroenteritis, showing that even normal microbiota can turn pathogenic under specific conditions.
Candida albicans is a fungus that causes infections such as candidiasis and, if systemic, can lead to pneumonia and sepsis. Additionally, the protozoan Plasmodium falciparum is the causative agent of malaria, a potentially life-threatening disease transmitted by mosquitoes. This diverse group of microorganisms exemplifies the variety of ways in which humans can interact with the microbial world, either through symbiosis or as hosts to pathogenic organisms.