What does germ theory explain regarding microorganisms?

Germ theory fundamentally explains that microorganisms, such as bacteria, viruses, fungi, and protozoa, can play diverse roles in the ecosystem, including both beneficial and harmful impacts on human health. The correct answer highlights that microorganisms contribute to food spoilage and can also be responsible for diseases in humans and other organisms. This theory, developed through the work of scientists like Louis Pasteur and Robert Koch, established a clear connection between specific pathogenic microorganisms and the diseases they cause, revolutionizing the understanding of infectious diseases.

Microorganisms are not solely responsible for disease; many play essential roles in processes such as fermentation, nutrient cycling, and maintaining health in ecosystems. For instance, certain bacteria are used in the production of yogurt and other fermented foods, illustrating their beneficial role. Additionally, they are vital in human digestion, particularly in the gut where they aid in breaking down food and synthesizing certain vitamins, which clarifies why options suggesting that microorganisms are only harmful or always harmful do not encompass the full spectrum of their impact.