Diseases caused by pathogenic organisms, viruses, bacteria, fungi, protozoa, or multicellular parasites. With the exception of viruses, the ease of treating the diseases depends on the extent of the difference between the metabolism of the pathogen and that of the host, so that infections with cestodes, nematodes, etc. can be more difficult to eliminate than those caused by bacteria, although the emergence of multidrug-resistant bacteria (see meticillin-resistant Staphylococcus aureus) in response to selection pressure by the use of antibiotics is a growing problem, particularly with nosocomial infections. How readily the disease spreads depends on infectivity, transmissibility (sometimes upon the numbers of vectors or carriers in the environment), and virulence (see epidemic; pandemic; zoonosis). Parasites such as schistosomes, trypanosomes, and Plasmodium can evade immune responses by antigenic variation; other organisms may be resistant because they occupy privileged sites within cells (e.g. various mycobacteria) or in the central nervous system (see Herpesviridae). Many of the common infectious diseases or agents have separate entries (e.g. encephalitis, hepatitis viruses). There are a range of defence mechanisms against infectious diseases, ranging from nonspecific mechanisms (see deleted in malignant brain tumours-1; Imd pathway; mannan-binding lectin; protein kinase R; pyrin domain proteins; toll-like receptors), phagocytic cells such as neutrophils and macrophages that use oxygen-dependent killing mechanisms and the more sophisticated immunoglobulin and cell mediated immune systems. Resistance to viral diseases depends upon nonspecific mechanisms such as interferons (see also cryopyrin; Mx proteins; oligoadenylate synthetases; RIG1; viperin) and on specific immunity, which can confer long-lasting resistance once a primary immune response has been mounted. Increasingly, antiviral drugs are being developed.
Subjects: Medicine and Health.