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Infection – is an invasion by and multiplication of pathogenic microscopic organisms, such as bacteria, viruses and fungi, in a bodily part or tissue of a host organism which may produce subsequent tissue injury and progress to overt disease through a variety of cellular or toxic mechanisms. The infecting organism, or pathogen, interferes with the normal functioning and perhaps the survival of the host organism.

Skin infections are common. Breaks in the skin integrity, particularly those that let in infectious agents(pathogens) like bacteria and fungi, frequently cause or aggravate skin infections. All skin infections can be divided into 3 classes by a disease-causing agent: The difference between an infection and colonization by microorganisms is often only a matter of circumstance. Organisms which are normally non-pathogenic and live on the surface of healthy skin can become pathogenic under the right conditions, and even the most infectious organism requires certain circumstances to cause a compromising infection. 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. The variables involved in the outcome of a host becoming inoculated by a pathogen and the ultimate outcome include:

  1. the route of entry of the pathogen and the access to host regions that it gains
  2. the intrinsic capacity of a microorganism to cause disease (virulence) of the particular organism
  3. the quantity or load of the initial substance introduced into the organism (inoculant)
  4. the immune status of the host being colonized
Any one who has a break in the skin is at risk for infection. However, certain conditions or diseases can put a person at greater risk for infection, including: diabetes (which causes poor blood flow to the skin), AIDS (because of a depressed immune system that is unable to fight the infection), skin damaged by sunburn or scratching.

Many invading organisms produce substances that cause allergic sensitivity in the host; the immune response to virus infection has been implicated in some diseases. 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.

Infection-causing agents:
  • Bacteria – are single-celled or noncellular spherical or spiral or rod-shaped organisms lacking chlorophyll that reproduce by fission; important as pathogens and for biochemical properties; taxonomy is difficult; often considered plants. Toxins, produced by bacteria, damage tissues of the host organism 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. Many bacteria such as some Staphylococcus species, Corynebacterium spp., Brevibacterium spp and Acinetobacter live on the surface of healthy skin, causing no harm. Propionibacteria live in the hair follicles of adult skin and contribute to acne. Some bacteria invade the skin and cause infection and may result in rashes (exanthems).
  • Virus – is a simple submicroscopic acellular organism consisting of a core of nucleic acid surrounded by protein that can grow and reproduce only inside living cells, making use of the machinery and metabolism of the hosting organism. For this reason, viruses are called obligate intracellular parasites. Before a virus has entered a host cell, it is called a virion – a package of viral genetic material. Virions can be passed from host to host either through direct contact or through a carrier (so-called vector).

    Viruses and retroviruses parasitize on host cells, causing cellular degeneration or cellular proliferation, as in warts and cold sores, which are caused by herpes simplex. Some viruses have been associated with the development of certain cancers (recently it has been shown that cervical cancer is caused at least partly by papillomavirus)
  • Fungi - (singular: fungus) are a major group of living things, originally considered plants lacking chlorophyll and leaves and true stems and roots and reproducing by spores, but now treated as the separate kingdom Fungi. They occur in all environments on the planet and include important decomposers and parasites. Parasitic fungi infect animals, including humans, other mammals, birds, and insects, with consequences varying from mild itching to death. In general, humans have a high level of innate immunity to fungi and most of the infections they cause are mild and self-limiting. This resistance is due to:
    1. Fatty acid content of the skin
    2. pH of the skin, mucosal surfaces and body fluids
    3. Epithelial turnover
    4. Normal flora
    5. Transferrin
    6. Cilia of respiratory tract.
    When fungi do pass the resistance barriers of the human body and establish infections, the infections are classified according to the tissue levels initially colonized. A fungal infection of the human body is called a mycosis. The most common of human mycoses is caused by dermatophytes. Although seen in all areas of the body one of the more common is tinea pedis or Athlete's foot. Mycoses are particularly severe in the case of immunodeficient patients, such as those suffering from AIDS.

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