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Protista

Single-celled eukaryotes of an enormous diversity are lumped together as the Kingdom Protista. Photosynthetic forms called phytoplankton, living in both fresh and saltwater, are probably responsible for more than half of all photosynthesis on earth. Some, particularly the ornate dinoflagellates, produce toxins and are responsible for shellfish poisoning when concentrated (biologuical magnification) by filterfeeding molluscs (clams, oysters, mussels) or directly for enormous fish kills when abundant (Red Tides, since the responsible organisms are red). Heterotrophic protists may directly absorb dissolved substances or may ingest particulates. Some protists are parasites, causing serious diseases in humans: amoebic dysentery, African sleeping sickness, malaria, Giardia; and similar diseases in other animals.

Viruses

Viruses are agents that infect cells, yet lack many of the characteristics of life. They have nucleic acid (DNA or RNA) shielded in a protein coat, but are incapable of metabolism, or self replication, depending on the host cells to carry out these functions. Viruses hijack the host cell and force it to synthesize more virus. As such, they are responsible for many severe diseases of other organisms, including bacteria, plants, and animals.

Best known are animal viruses such as: Which produce Diseases:
DNA Viruses
Adenoviruses Respiratory infections; pinkeye
Parvoviruses Gasteroenteritis (diarrhea, vomiting)
Papovaviruses Warts in humans; cancer in mice
Herpesviruses Fever blisters; chickenpox; genital infections; one form of cancer
Poxviruses Smallpox; cowpox; benign tumors
RNA Viruses
Enteroviruses Diarrhea; polio; aseptic meningitis
Rhinoviruses Common colds
Togaviruses Yellow fever; German measles
Influenzaviruses influenza
Paramyxoviruses Respiratory disorders; measles
Rhabdoviruses Rabies
Arenaviruses Meningitis; hemorrhagic fevers
Coronaviruses Upper respiratory disease
Retroviruses Tumors; lukemia; AIDS
Reoviruses Mild respiratory disorders; severe diarrhea in humans, cattle, mice