There are other infectious entities which are known to cause disease in humans and other animals, and which do not fall under any of the taxonomic group or class of microorganisms including bacteria, fungi, algae, protozoa, Archaea and viruses. These infectious bodies are usually very small and sub-cellular in nature. They are usually smaller than the known viruses that cause infectious diseases in humans. These other infectious entities cause very rare, miscellaneous and fatal but progressive degenerative diseases in both humans and animals. They lack nucleic acids (excluding viroids which are made up of single-stranded RNA), and are majorly made up of infectious protein particles that primarily affect the central nervous system (CNS) of their hosts (humans and animals inclusive).
Prions and viroids are examples of these other infectious bodies which are quite different from viruses, bacteria, fungi, protozoa and algae. Viroids are small, single-stranded (ss) circular RNA molecules that chiefly cause diseases in plants. Prions are proteinous molecules which are neither viruses nor viroids, and which cause a range of diseases in humans (e.g. Creutzfeldt-Jacob disease, CJD) and animals (e.g. scrapie and bovine spongiform encephalopathy, BSE). Prions are subcellular infectious entities that lack nucleic acids, and they cause a range of degenerative disease that affects parts of the central nervous system (CNS) of both animals and man.
PATHOGENIC ANIMALS THAT CAUSE INFECTIOUS DISEASES IN HUMANS
Infectious diseases in humans and other animals are majorly caused by infectious particles (i.e. microorganisms which are too small to be seen by the naked eyes) which are generally termed pathogenic microorganisms including bacteria, viruses, fungi and protozoa. But they also exists other infectious bodies some of which can be seen under the microscope and which can also be visible by the naked eyes because of their presumed larger forms. These pathogenic animals are referred to as parasitic worms (e.g., helminthes) and arthropods (e.g., mites).
Helminthes and arthropods are invertebrates, and they are found in the Kingdom Animalia. They are known to cause a variety of infectious diseases in humans and animals. Arthropods which may include insects usually serve as vectors which help to transmit pathogenic microorganisms (e.g. bacteria) to humans usually through a bite. Microorganisms can also reach humans and cause disease through arthropods when they come in direct contact with their food or defecate on the body of humans. Helminthes includes all parasitic worms such as roundworms (nematodes), tapeworms (cestodes), and flatworms or flukes (which are trematodes), and they are known to cause a variety of gastrointestinal infections in humans.
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