Viroid in the context of "Infectious agents"

In biology, a pathogen (Greek: πάθος, pathos "suffering", "passion" and -γενής, -genēs "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ.

The term pathogen came into use in the 1880s. Typically, the term pathogen is used to describe an infectious microorganism or agent, such as a virus, bacterium, protozoan, prion, viroid, or fungus. Small animals, such as helminths and insects, can also cause or transmit disease. However, these animals are usually referred to as parasites rather than pathogens. The scientific study of microscopic organisms, including microscopic pathogenic organisms, is called microbiology, while parasitology refers to the scientific study of parasites and the organisms that host them.

An RNA virus is a virus characterized by a ribonucleic acid (RNA) based genome. The genome can be single-stranded RNA (ssRNA) or double-stranded (dsRNA). Notable human diseases caused by RNA viruses include influenza, SARS, MERS, COVID-19, Dengue virus, hepatitis C, hepatitis E, West Nile fever, Ebola virus disease, rabies, polio, mumps, and measles.

All RNA viruses use a homologous RNA-dependent polymerase for replication and are categorized by the International Committee on Taxonomy of Viruses (ICTV) into the realm Riboviria. This includes viruses belonging to Group III, Group IV, Group V, and Group VI of the Baltimore classification system. Group VI comprises the retroviruses, which have RNA genetic material but use DNA intermediates in their life cycle. Riboviria does not include viroids and satellite nucleic acids: Deltavirus, Avsunviroidae, and Pospiviroidae are taxa that were mistakenly included in 2019, but this was corrected in 2020.

Plant diseases are diseases in plants caused by pathogens (infectious organisms) and environmental conditions (physiological factors). Organisms that cause infectious disease include fungi, oomycetes, bacteria, viruses, viroids, virus-like organisms, phytoplasmas, protozoa, nematodes and parasitic plants. Not included are ectoparasites like insects, mites, vertebrates, or other pests that affect plant health by eating plant tissues and causing injury that may admit plant pathogens. The study of plant disease is called plant pathology.