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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 healthyskin, 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:
Source: National Skin Care Institute