Pathogenic bacteria are primarily known for causing what?

Pathogenic bacteria are primarily known for causing infections. These microorganisms invade the body and can multiply, leading to a range of illnesses depending on the type of bacteria and the area of the body they affect. They can cause severe health issues, from mild infections like strep throat to more severe conditions such as pneumonia or sepsis. Understanding the role of pathogenic bacteria is crucial in fields like healthcare and cosmetology, particularly in nail and skin care, where hygiene practices are essential to prevent infections.

The other options, while related to the immune system and health, do not directly describe the main outcome of pathogenic bacteria. Allergic reactions are typically caused by the body's response to benign substances, immune responses involve the action of the body's defenses against pathogens, and environmental changes are often associated with bacteria in general but not specifically tied to pathogenic strains. Thus, infections remain the most accurate and direct consequence linked to pathogenic bacteria.