Which type of pathogens can cause disease only when the host's immune system is weakened?

Opportunistic pathogens are a specific group of microorganisms that typically do not cause disease in a healthy host with a fully functioning immune system. Instead, they take advantage of a weakened immune response to establish infection and cause disease. This can occur in individuals with compromised immune systems due to various factors, such as chronic illnesses, malnutrition, immunosuppressive therapy, or certain genetic conditions.

For instance, common opportunistic pathogens include certain strains of bacteria like Pseudomonas aeruginosa, fungi such as Candida species, and even some viruses. In a healthy person, these organisms may reside harmlessly or exist symbiotically without causing any harm. However, when the body's defenses are down, they can proliferate and lead to opportunistic infections.

The other types of pathogens listed do not fit this definition. Obligate pathogens are those that are always associated with disease regardless of the host's immune status. Environmental pathogens are typically organisms found in the environment that can infect healthy individuals under certain conditions but do not solely rely on an immune-compromised state. Infectious pathogens is a broader term that can include both obligate and opportunistic pathogens, but it does not specifically denote the characteristic of only causing disease in the context of a weakened immune system