Which type of organisms causes diseases in humans and are single-celled?

The correct answer focuses on parasitic protozoa, which are indeed single-celled organisms known for causing diseases in humans. Protozoa belong to the Kingdom Protista and are characterized by their unicellular structure. They can invade host organisms and disrupt normal biological functions, leading to infections and disease. Common diseases caused by parasitic protozoa include malaria, amoebic dysentery, and giardiasis.

While bacterial cells are also single-celled and can cause diseases, the question specifically seeks the type of organism that is classified as a parasite, which narrows the scope to parasitic protozoa. Fungi, though they can be single-celled (as in yeasts) or multicellular (as in molds), are not classified as pathogens in the same category as protozoa and are generally considered more complex organisms. Helminths, on the other hand, are multicellular parasitic organisms (worms) and do not fit the criterion of being single-celled. Thus, the specific identification of a single-celled parasite confirms parasitic protozoa as the appropriate answer.