What term describes unicellular organisms that can cause disease?

The term that describes unicellular organisms that can cause disease is bacteria. Bacteria are single-celled microorganisms that can thrive in various environments and play diverse roles in their ecosystems. While many bacteria are harmless or beneficial, some species have pathogenic capabilities and can lead to diseases in humans, animals, and plants. They do this through various mechanisms, including producing toxins or invading host tissues.

In the context of this question, other choices include viruses, which are not considered unicellular organisms as they require a host cell to replicate and do not possess cellular structures. Fungi consist of single-celled or multicellular organisms, but many fungi are not unicellular or do not fit the primary definition of causing disease in the same way as bacteria. Parasites, while capable of causing disease, typically refer to multicellular organisms that depend on a host for survival, distinguishing them from the strictly unicellular nature of bacteria.