Answer:
Viruses: Viruses are the smallest infectious agents and are composed of genetic material (DNA or RNA) surrounded by a protein coat. They are not capable of replicating on their own and must hijack the machinery of a host cell in order to replicate and cause infection. Examples of viruses that cause disease in humans include the influenza virus, HIV, and the coronavirus.
Bacteria: Bacteria are single-celled organisms that can reproduce on their own. They are typically larger than viruses and are composed of a cell wall, cytoplasm, and genetic material. Bacteria can be responsible for a range of infectious diseases including strep throat, food poisoning, and tuberculosis.
Protists: Protists are single-celled organisms that can be either free-living or parasitic. They are larger than viruses and bacteria and are composed of a cell wall, cytoplasm, and genetic material. Examples of protists that cause human disease are the malaria parasite and Giardia lamblia.
Fungus: Fungi are eukaryotic organisms that can be either single-celled or multi-celled. Fungi are composed of cell walls, cytoplasm, and genetic material. Examples of fungal diseases in humans include athlete's foot, ringworm, and candidiasis.
Parasites: Parasites are organisms that live in or on another organism, called the host, and derive their nutrition from the host. Examples of parasites that cause human disease include the roundworm, tapeworm, and malaria-causing Plasmodium parasites.