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Infectious diseases are caused by the invasion of an organism's tissues by pathogenic microorganisms, such as bacteria, viruses, fungi, protozoa or other parasites and prions. The study of these microorganisms is known as microbiology. Upon infection by a pathogen the body mounts a protective immune response, which is either innate, i.e. non-specific, or adaptive, i.e. acquired and highly specific.
Infections can spread from person to person via direct or indirect transmission. Examples of direct transmission include via exchange of bodily fluids from sexual contact or blood transfusion. Infections can be indirectly transmitted via air borne droplets, as a result of sneezing, coughing or talking, or from an inanimate object that has previously been contaminated with a pathogen. In addition, some infections are transmitted from animal to person (zoonotic), such as ebola, swine flu (H1N1), rabies and COVID-19 (SARS-CoV-2).
The immune system is a protective mechanism which responds to pathogenic invasion by mounting an immune response. It can discriminate between self and foreign antigens; this enables an organism to maintain homeostasis. If this ability to discriminate is disrupted, the immune system can initiate an autoimmune reaction against cells or tissues of the body, which can develop into an autoimmune disease.
Innate immunity is an organism's first line of defence against invasion by any pathogen, including those not previously encountered and is not specific to a particular invader. The innate immune system consists of physical barriers, such as the skin and mucus membranes, which block entry of foreign particles. If these surfaces are penetrated, immune cells, including monocytes, macrophages, neutrophils, dendritic cells and natural killer cells, are activated which respond by phagocytosis of the invader and release of cytokines and complement to recruit further cells and amplify the inflammatory response. Innate immunity is evolutionarily conserved.
Adaptive or acquired immunity is an organism's response to a specific antigen, pathogen or immunogen and is triggered by the body's recognition of an invader that it has encountered previously. The adaptive immune system consists of an antibody response and a cell-mediated response. The cells comprising the adaptive immune system are known as lymphocytes, of which there are 2 types: B cells and T cells. Upon activation B cells release antibodies which bind specific antigens, inactivating viruses or bacterial toxins and targeting them for destruction by the innate immune system. Some T cells, known as CD8+, act as killer cells, directly attack virus-infected cells that present viral antigens on the surface. Other T cell populations known as CD4+ or helper cells (Th), can signal to other immune cells such as macrophages to destroy an invading pathogen. A third population T cells, termed regulatory T cells or Tregs, distinguish alien cells from self, preventing the immune system from mounting an inappropriate immune response. However, Tregs also have a role in cancer as they can prevent the immune system from recognizing tumor cells as abnormal.