What are single-celled organisms that can cause disease, often found in various environments?

Single-celled organisms that can cause disease are primarily classified as protozoa. These microscopic organisms are known for their ability to live in diverse environments, including freshwater, saltwater, soil, and as parasites within hosts. Protozoa can cause a range of diseases in humans and animals, such as malaria, amoebic dysentery, and giardiasis, by invading host cells or tissues.

In contrast, helminths are multicellular organisms (worms) that can also cause disease but are not single-celled. Fungi, although they can be single-celled (like yeast), include multicellular forms and typically do not fit the definition as a primary causative agent for many diseases, especially in the context of single-celled pathogens. Mycoplasmas are a unique group of bacteria that lack a cell wall, but they are not classified as single-celled organisms in the same context as protozoa. Thus, the most accurate answer identifying single-celled infectious agents that can cause disease is protozoa.