Infection is the invasion of a host organism's bodily tissues by disease-causing organisms, their multiplication, and the reaction of host tissues to these organisms and the toxins they produce. Infections are caused by microorganisms such as viruses, prions, bacteria, and viroids, and larger organisms like macroparasites and fungi.
Hosts can fight infections using their immune system. Mammalian hosts react to infections with an innate response, often involving inflammation, followed by an adaptive response. Pharmaceuticals can also help fight infections.
Symptomatic infections are apparent, whereas an infection that is active, but does not produce noticeable symptoms, may be called inapparent, silent, or subclinical. An infection that is inactive or dormant is called a latent infection.
Primary and secondary infections
Primary and secondary infection may either refer to succeeding infections or different stages of one and the same infection such as in acute herpes labialis infection.[dubious ] In the latter case, acute infection may also be used, as in acute HIV infection.
Occult infection is a hidden infection first recognized by secondary manifestations. Fran Giampietro discovered this type, and coined the term "occult infection" in the late 1930s.
Some signs are specifically characteristic and indicative of a disease and are called pathognomic signs; but these are rare. If an infection is suspected, blood, urine and sputum cultures are usually the first step. Chest x-rays and stool analysis may also aid diagnosis. Spinal fluid can be tested to ensure that there is no brain infection.
In children the presence of cyanosis, rapid breathing, poor peripheral perfusion, or a petechial rash increases the risk of a serious infection by greater than 5 fold. Other important indicators include parental concern, clinical instinct, and temperature greater than 40 °C.
Signs and symptoms
The symptoms of an infection depend on the type of disease. Some signs of infection affect the whole body generally, such as fatigue, loss of appetite, weight loss, fevers, night sweats, chills, aches and pains. Others are specific to individual body parts, such as skin rashes, coughing, or a runny nose.
Bacterial and viral infections can both cause the same kinds of symptoms. It can be difficult to distinguish which is the cause of a specific infection. It's important to distinguish, because viral infections cannot be cured by antibiotics.
|Characteristic||Viral infection||Bacterial infection|
|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. 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 is infected with bacteria, pain occurs at the site of the infection. Bacterial throat pain is often characterized by more pain on one side of the throat. An ear infection is more likely to be diagnosed as bacterial if the pain occurs in only one ear. A cut that produces pus and milky-colored liquid is most likely infected.|
|Cause||Pathogenic viruses||Pathogenic bacteria|
There is a general chain of events that applies to infections. For infections to occur, a given chain of events must occur. The chain of events involves several steps—which include the infectious agent, reservoir, entering a susceptible host, exit and transmission to new hosts. Each of the links must be present in a chronological order for an infection to develop. Understanding these steps helps health care workers target the infection and prevent it from occurring in the first place.
Infection begins when an organism successfully colonizes by entering the body, growing and multiplying. Most humans are not easily infected. Those who are weak, sick, malnourished, have cancer or are diabetic have increased susceptibility to chronic or persistent infections. Individuals who have a suppressed immune system are particularly susceptible to opportunistic infections. Entrance to the host generally occurs through the mucosa in orifices like the oral cavity, nose, eyes, genitalia, anus, or open wounds. While a few organisms can grow at the initial site of entry, many migrate and cause systemic infection in different organs. Some pathogens grow within the host cells (intracellular) whereas others grow freely in bodily fluids.
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 is the anaerobic bacteria species, which colonizes the mammalian colon, and an example of the latter is various species of staphylococcus that exist on human skin. Neither of these colonizations are considered infections. The difference between an infection and a colonization is often only a matter of circumstance. Non-pathogenic organisms can become pathogenic given specific 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 remains harmless on the skin, but, when present in a normally sterile space, such as in the capsule of a joint or the peritoneum, multiplies without resistance and creates a burden on the host.
It can be difficult to know which chronic wounds are infected. Despite the huge number of wounds seen in clinical practice, there are limited quality data for evaluated symptoms and signs. A review of chronic wounds in the Journal of the American Medical Association's "Rational Clinical Examination Series" quantified the importance of increased pain as an indicator of infection. The review showed that the most useful finding is an increase in the level of pain [likelihood ratio (LR) range, 11-20] makes infection much more likely, but the absence of pain (negative likelihood ratio range, 0.64-0.88) does not rule out infection (summary LR 0.64-0.88).
Disease can arise if the host's protective immune mechanisms are compromised and the organism inflicts damage on the host. Microrganisms can cause tissue damage by releasing a variety of toxins or destructive enzymes. For example, Clostridium tetani releases a toxin that paralyzes muscles, and staphylococcus releases toxins that produce shock and sepsis. Not all infectious agents cause disease in all hosts. For example less than 5% of individuals infected with polio develop disease. On the other hand, some infectious agents are highly virulent. The prion causing mad cow disease and Creutzfeldt–Jakob disease kills almost all animals and people that are infected.
Persistent infections occur because the body is unable to clear the organism after the initial infection. Persistent infections are characterized by the continual presence of the infectious organism, often as latent infection with occasional recurrent relapses of active infection. There are some viruses that can maintain a persistent infection by infecting different cells of the body. Some viruses once acquired never leave the body. A typical example is the herpes virus, which tends to hide in nerves and become reactivated when specific circumstances arise.
Persistent infections cause millions of deaths globally each year. Chronic infections by parasites account for a high morbidity and mortality in many underdeveloped countries.
For infecting organisms to survive and repeat the infection cycle in other hosts, they (or their progeny) must leave an existing reservoir and cause infection elsewhere. Infection transmission can take place via many potential routes. Infectious organisms may be transmitted either by direct or indirect contact. Direct contact occurs when an individual comes into contact with the reservoir. This may mean touching infected bodily fluids or drinking contaminated water or being bitten by the deer tick. Direct contact infections can also result from inhalation of infectious organisms found in aerosol particles emitted by sneezing or coughing. Another common means of direct contact transmission involves sexual activity - oral, vaginal, or anal sex.
Indirect contact occurs when the organism is able to withstand the harsh environment outside the host for long periods of time and still remain infective when specific opportunity arises. Inanimate objects that are frequently contaminated include toys, furniture, door knobs, tissue wipes or personal care products from an infected individual. Consuming food products and fluid that have been contaminated by contact with an infecting organism is another case of disease transmission by indirect contact.
A common method of transmission in under developed countries is fecal-oral transmission. In such cases, sewage water is used to wash food or is consumed. This results in food poisoning. Common fecal-oral transmitted pathogens include Vibrio cholerae, Giardia species, rotaviruses, Entameba histolytica, Escherichia coli, and tape worms. Most of these pathogens cause gastroenteritis.
All the above modes are examples of horizontal transmission because the infecting organism is transmitted from person to person in the same generation. There are also a variety of infections transmitted vertically - that is from mother to child during the birthing process or fetal development. Common disorders transmitted this way include AIDs, hepatitis, herpes, and cytomegalovirus 
Treatment and prevention
Viable treatment and prevention strategies disrupt the infection cycle. For example, direct transmission can be diminished by adequate hygiene, maintaining a sanitary environment, and health education.
When infection attacks the body, anti-infective drugs can suppress the infection. Four types of anti-infective or drugs exist: antibacterial (antibiotic), antiviral, antitubercular, and antifungal. Depending on the severity and the type of infection, the antibiotic may be given by mouth, injection or may be applied topically. Severe infections of the brain are usually treated with intravenous antibiotics. Sometimes, multiple antibiotics are used to decrease the risk of resistance and increase efficacy. Antibiotics only work for bacteria and do not affect viruses. Antibiotics work by slowing down the multiplication of bacteria or killing the bacteria. The most common classes of antibiotics used in medicine include penicillin, cephalosporins, aminoglycosides, macrolides, quinolones and tetracyclines.
Techniques like hand washing, wearing gowns, and wearing face masks can help prevent infections from being passed from the surgeon to the patient or vice versa. Frequent hand washing remains the most important defense against the spread of unwanted organisms. Nutrition must be improved and one has to make changes in life style- such as avoiding the use of illicit drugs, using a condom, and entering an exercise program. Cooking foods well and avoiding foods that have been left outside for a long time is also important. Do not take antibiotics for longer than needed. Long term use of antibiotics leads to resistance and chances of developing opportunistic infections like clostridium difficile colitis.Vaccination is another means of preventing infections by facilitating the development of immune resistance in vaccinated hosts.
Evidence of infection in fossil remains is a subject of interest for paleopathologists, scientists who study occurrences of injuries and illness in extinct life forms. Signs of infection have been discovered in the bones of carnivorous dinosaurs. When present, however, these infections seem to tend to be confined to only small regions of the body. A skull attributed to the early carnivorous dinosaur Herrerasaurus ischigualastensis exhibits pit-like wounds surrounded by swollen and porous bone. The unusual texture of the bone around the wounds suggests they were afflicted by a short-lived, non-lethal infection. Scientists who studied the skull speculated that the bite marks were received in a fight with another Herrerasaurus. Other carnivorous dinosaurs with documented evidence of infection include Acrocanthosaurus, Allosaurus, Tyrannosaurus and a tyrannosaur from the Kirtland Formation. The infections from both tyrannosaurs were received by being bitten during a fight, like the Herrerasaurus specimen.
Notes and references
- Definition of "infection" from several medical dictionaries - Retrieved on 2012-04-03
- Kayser, Fritz H; Kurt A Bienz; Johannes Eckert; Rolf M Zinkernagel (2005). Medical microbiology. Stuttgart: Georg Thieme Verlag. p. 398. ISBN 3-13-131991-7.
- Van den Bruel A, Haj-Hassan T, Thompson M, Buntinx F, Mant D (March 2010). "Diagnostic value of clinical features at presentation to identify serious infection in children in developed countries: a systematic review". Lancet 375 (9717): 834–45. doi:10.1016/S0140-6736(09)62000-6. PMID 20132979.
- Bacterial vs. Viral Infections - Do You Know the Difference? National Information Program on Antibiotics
- Infection Cycle - Retrieved on 2010-01-21
- Understanding Infectious Diseases Science.Education.Nih.Gov article - Retrieved on 2010-01-21
- Reddy M, Gill SS, Wu W, et al. Does this patient have an infection of a chronic wound?. JAMA. 2012 Feb 8;307(6):605-11. (Review) PMID 22318282
- Chronic Infection Information Retrieved on 2010-01-14
- What is Animal virus? www.ecomii.com - Retrieved on 2010-01-21
- Intestinal Parasites and Infection fungusfocus.com - Retrieved on 2010-01-21
- Virology and AIDS Virology and Human Immunodeficiency Virus - Retrieved on 2010-01-21
- Nursing Pharmacology Made Incredibly Easy. Lippincott Williams & Wilkins. 2009.p.483
- Bacterial Infection Causes 2010-02-08
- World Health Organization. "Staphylococcal infection" 2010-02-08.
- Generalized Infectious Cycle Diagram Illustration - Retrieved on 2010-01-21
- eMedicine Health. "Bacterial and Viral Infections" 2010-02-08.
- Molnar, R. E., 2001, Theropod paleopathology: a literature survey: In: Mesozoic Vertebrate Life, edited by Tanke, D. H., and Carpenter, K., Indiana University Press, p. 337-363.
- Vaccine Research Center Information concerning vaccine research clinical trials for Emerging and re-Emerging Infectious Diseases.
- Infection Information Resource
- Knowledge source for Health Care Professionals involved in Wound management www.woundsite.info
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