Invading organisms such as bacteria produce toxins that damage host tissues and interfere with normal metabolism; some toxins are actually enzymes that, by breaking down host tissues, prevent the localization of infections. Other bacterial substances destroy the host's phagocytes. Viruses and retroviruses are parasitic on host cells, causing cellular degeneration, as in rabies, poliomyelitis, and AIDS, or cellular proliferation, as in warts and cold sores. Some viruses have been associated with the development of certain cancers. Substances produced by many invading organisms cause allergic sensitivity in the host; the immune response to virus infection has been implicated in some diseases (see allergy).
Infections may be spread via respiratory droplets, direct contact, contaminated food, or vectors, such as insects. They can also be transmitted sexually (see sexually transmitted diseases) and from mother to fetus. Immunity is the term used to describe the capacity of the host to respond to infection. Drugs that help fight infections include antibiotics and antiviral drugs.
See also specific diseases, diseases of plants.
See J. Waller, The Discovery of the Germ (2003).
Invasion of the body by various agents—including bacteria, fungi (see fungus), protozoans, viruses, and worms—and its reaction to them or their toxins. Infections are called subclinical until they perceptibly affect health, when they become infectious diseases. Infection can be local (e.g., an abscess), confined to one body system (e.g., pneumonia in the lungs), or generalized (e.g., septicemia). Infectious agents can enter the body by inhalation, ingestion, sexual transmission, passage to a fetus during pregnancy or birth, wound contamination, or animal or insect bites. The body responds with an attack on the invader by leukocytes, production of antibodies or antitoxins, and often a rise in temperature. The antibodies may result in short-term or lifelong immunity. Despite significant progress in preventing and treating infectious diseases, they remain a major cause of illness and death, particularly in regions of poor sanitation, poor nutrition, and crowding.
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A secondary infection is an infection that occurs during or following treatment of another already existing primary infection.
The variables involved in the outcome of a host becoming inoculated by a pathogen and the ultimate outcome include:
As an example, the staphylococcus species present on skin remain harmless on the skin, but, when present in a normally sterile space, such as in the capsule of a joint or the peritoneum, will multiply without resistance and create a huge burden on the host.
|Typical symptoms||In general, viral infections are systemic. This means they involve many different parts of the body or more than one body system at the same time; i.e. a runny nose, sinus congestion, cough, body aches etc. They can be local at times as in viral conjunctivitis or "pink eye" and herpes. Only a few viral infections are painful, like herpes. The pain of viral infections is often described as itchy or burning.,||The classic symptoms of a bacterial infection are localized Redness, Heat, Swelling and Pain. In ancient Rome the terms were; Rubor, Calor, Turgor, and Dolor. One of the hallmarks of a bacterial infection is local pain, pain that is in a specific part of the body. For example, if a cut occurs and it is infected with bacteria, pain will occur at the site of the infection. Bacterial throat pain is often characterised by more pain on one side of the throat. An ear infection is more likely to be bacterial if the pain occurs in only one ear. An infection that produces pus is not always bacterial.|
|Cause||Pathogenic viruses||Pathogenic bacteria|