In biology, a pathogen in the oldest and broadest sense, is anything that can produce disease. A pathogen may be referred to as an infectious agent, or a germ; the term pathogen came into use in the 1880s. The term is used to describe an infectious microorganism or agent, such as a virus, protozoan, viroid, or fungus. Small animals, such as certain kinds of worms and insect larvae, can produce disease. However, these animals are in common parlance, referred to as parasites rather than pathogens; the scientific study of microscopic organisms, including microscopic pathogenic organisms, is called microbiology, while the study of disease that may include these pathogens is called pathology. Parasitology, meanwhile, is the scientific study of parasites and the organisms that host them. There are several pathways; the principal pathways have different episodic time frames, but soil has the longest or most persistent potential for harboring a pathogen. Diseases in humans that are caused by infectious agents are known as pathogenic diseases, though not all diseases are caused by pathogens.

Some diseases, such as Huntington's disease, are caused by inheritance of abnormal genes. Pathogenicity is the potential disease-causing capacity of pathogens. Pathogenicity is related to virulence in meaning, but some authorities have come to distinguish it as a qualitative term, whereas the latter is quantitative. By this standard, an organism may be said to be pathogenic or non-pathogenic in a particular context, but not "more pathogenic" than another; such comparisons are described instead in terms of relative virulence. Pathogenicity is distinct from the transmissibility of the virus, which quantifies the risk of infection. A pathogen may be described in terms of its ability to produce toxins, enter tissue, hijack nutrients, its ability to immunosuppress the host, it is common to speak of an entire species of bacteria as pathogenic when it is identified as the cause of a disease. However, the modern view is. A bacterium may participate in opportunistic infections in immunocompromised hosts, acquire virulence factors by plasmid infection, become transferred to a different site within the host, or respond to changes in the overall numbers of other bacteria present.

For example, infection of mesenteric lymph glands of mice with Yersinia can clear the way for continuing infection of these sites by Lactobacillus by a mechanism of "immunological scarring". Virulence evolves when a pathogen can spread from a diseased host, despite the host becoming debilitated. Horizontal transmission occurs between hosts of the same species, in contrast to vertical transmission, which tends to evolve toward symbiosis by linking the pathogen's evolutionary success to the evolutionary success of the host organism. Evolutionary biology proposes that many pathogens evolve an optimal virulence at which the fitness gained by increased replication rates is balanced by trade-offs in reduced transmission, but the exact mechanisms underlying these relationships remain controversial. Transmission of pathogens occurs through many different routes, including airborne, direct or indirect contact, sexual contact, through blood, breast milk, or other body fluids, through the fecal-oral route.

Prions are misfolded proteins that can transfer their misfolded state to other folded proteins of the same type. They do not contain any DNA or RNA and cannot replicate other than to convert existing normal proteins to the misfolded state; these abnormally folded proteins are found characteristically in some diseases such as scrapie, bovine spongiform encephalopathy and Creutzfeldt–Jakob disease. Viruses are small particles between 20 and 300 nanometers in length, containing RNA or DNA. Viruses require a host cell to replicate; some of the diseases that are caused by viral pathogens include smallpox, mumps, chickenpox, ebola, HIV, rubella. Pathogenic viruses are from the families: Adenoviridae, Herpesviridae, Flaviviridae, Orthomyxoviridae, Papovaviridae, Polyomavirus and Togaviridae. HIV is a notable member of the family Retroviridae which affected 37.9 million people across the world in 2018. The vast majority of bacteria, which can range between 0.15 to 700 μM in length, are harmless or beneficial to humans.

However, a small list of pathogenic bacteria can cause infectious diseases. Pathogenic bacteria have several ways, they can either directly affect the cells of their host, produce endotoxins that damage the cells of their host, or cause a strong enough immune response that the host cells are damaged. One of the bacterial diseases with the highest disease burden is tuberculosis, caused by the bacterium Mycobacterium tuberculosis, which killed 1.5 million people in 2013 in sub-Saharan Africa. Pathogenic bacteria contribute to other globally significant diseases, such as pneumonia, which can be caused by bacteria such as Streptococcus and Pseudomonas, foodborne illnesses, which can be caused by bacteria such as Shigella and Salmonella. Pathogenic bacteria cause infections such as tetanus, typhoid fever, diphtheria and leprosy. Fungi are eukaryotic organisms. There are 300 known fungi that are pathogenic to humans including Candida albicans, th

Ettore Mazzoleni

Ettore Mazzoleni was a Canadian conductor, music educator and arts administrator of Swiss birth. He was one of the Canadian Opera Company's principal conductors during its early years, working there from 1958 to 1968. For many years he was the program annotator of the Toronto Symphony Orchestra and served as the orchestra's associate conductor from 1942–1948, he became a naturalized Canadian citizen in 1949. Born to Swiss-Italian parents in Brusio, Switzerland, Mazzoleni earned a Bachelor of Music and a Bachelor of Arts in mathematics from the University of Oxford in 1927, he went on to study piano at the Royal College of Music while working on the school's opera staff from 1927–1929. While there he had the opportunity to work with Sir Adrian Boult and Ralph Vaughan Williams. In 1929 Mazzoleni immigrated to Canada to join the music and English faculties of the Upper Canada College, where he remained until 1945, he frequently worked as a consultant for the opera program at the Toronto Conservatory of Music, beginning with the school's 1929 production of Vaughan Williams's Hugh the Drover.

In 1932 he joined the TCM's music faculty where he taught music conducting. He succeeded Donald Heins as the director of the TCM's symphony orchestra in 1934. While still teaching at the TCM, Mazzoleni was appointed the director of the Opera Division at the University of Toronto in 1952, a post he held until 1966. During his tenure he worked with the Opera Festival of Toronto, founded by the UTOD in 1950. In 1958 the OFT became a full-time professional opera company, the Canadian Opera Company, Mazzoleoni was active conducting operas with the company until his sudden death in a car accident in 1968. Among his notable pupils were musicians Howard Cable, Robert Fleming, James Gayfer, Godfrey Ridout, Rudy Toth

Carl Justi

Carl Justi was a German art historian, who practised a biographical approach to art history. Professor of art history at the University of Bonn, he wrote three major critical biographies: of Johann Joachim Winckelmann, of Diego Velázquez and of Michelangelo. Born in Marburg, Justi studied theology at the University of Berlin before transferring to philosophy, he graduated in 1859 with a thesis'Über die ästhetischen Elemente in der platonischen Philosophie'. Justi established his reputation with a three-volume work on Johann Joachim Winckelmann, he succeeded Anton Springer in the chair of art history at the University of Bonn, holding the post from 1872 until 1901. Die ästhetischen Elemente in der platonischen Philosophie: ein historisch-philosophischer Versuch, Marburg: N. G. Elwert, 1860 Winckelmann: sein Leben, Seine Werke und sein Zeitgenossen. 3 vols. Leipzig: F. C. W. Vogel, 1866-72. Diego Velázquez und sein Jahrhundert. Bonn: M. Cohen, 1888. Translated into English as Diego Velázquez and His Times.

London: H. Grevel, 1889. Michelangelo: Beiträge zur Erklärung der Werke und des Menschen. Leipzig: Breitkopf & Härtel, 1900 Carl Justi at the Dictionary of Art Historians