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An infection is the detrimental colonization of a host organism by a foreign species. In an infection, the infecting organism seeks to utilize the host's resources to multiply (usually at the expense of the host). The infecting organism, or pathogen, interferes with the normal functioning of the host and can lead to chronic wounds, gangrene, loss of an infected limb, and even death. The host's response to infection is inflammation. Colloquially, a pathogen is usually considered a microscopic organism though the definition is broader, including feces, parasites, fungi, viruses, prions, and viroids. A symbiosis between parasite and host, whereby the relationship is beneficial for the former but detrimental to the latter, is characterised as parasitism. The branch of medicine that focuses on infections and pathogens is infectious disease.

A secondary infection is an infection that occurs during or following treatment of another already existing primary infection.


Wound colonization refers to nonreplicating microorganisms within the wound, while in infected wounds replicating organisms exist and tissue is injured. All multicellular organisms are colonized to some degree by extrinsic organisms, and the vast majority of these exist in either a mutualistic or commensal relationship with the host. An example of the former would be the anaerobic bacteria species which colonize the mammalian colon, and an example of the latter would be the various species of staphylococcus which exist on human skin. Neither of these colonizations would be considered infections. The difference between an infection and a colonization is often only a matter of circumstance. Organisms which are non-pathogenic can become pathogenic under the right conditions, and even the most virulent organism requires certain circumstances to cause a compromising infection. Some colonizing bacteria, such as Corynebacteria sp. and viridans streptococci, prevent the adhesion and colonization of pathogenic bacteria and thus have a symbiotic relationship with the host, preventing infection and speeding wound healing.

The variables involved in the outcome of a host becoming inoculated by a pathogen and the ultimate outcome include:

  • the route of entry of the pathogen and the access to host regions that it gains
  • the intrinsic virulence of the particular organism
  • the quantity or load of the initial inoculant
  • the immune status of the host being colonized

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.

Occult infection

An occult infection is medical terminology for a "hidden" infection, that is, one which presents no symptoms. Dr. Fran Giampietro discovered this type, and coined the term "occult infection" in the late 1930s.

Bacterial or viral

Bacterial and viral infections can both cause similar symptoms such as malaise, fever, and chills. It can be difficult, even for a doctor to distinguish which is the cause of a specific infection. It's important to distinguish, because viral infections cannot be cured by antibiotics.

Comparison of viral and bacterial infection
Characteristic Viral Bacterial
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
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