An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease. Colloquially, infections are usually considered to be caused by microscopic organisms or microparasites like viruses, prions, bacteria, and viroids, though larger organisms like macroparasites and fungi can also infect.
Hosts normally fight infections themselves via 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.
The branch of medicine that focuses on infections and pathogens is infectious disease medicine.
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Infections are classified in multiple ways. They are classified by the causative agent as well as by the constellation of symptoms and medical signs that are produced.
An infection that produces symptoms is an apparent infection. 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.[1]
A short-term infection is an acute infection. A long-term infection is a chronic infection.
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. In the latter case, acute infection may also be used, as in acute HIV infection.
An occult or asymptomatic infection is medical terminology for a "hidden" infection, that is, one which presents no symptoms verifiable and recognizable by a doctor. Dr. Fran Giampietro discovered this type, and coined the term "occult infection" in the late 1930s.
Diagnosis of infections can be difficult as specific signs and symptoms 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.[2] Other important indicators include parental concern, clinical instinct, and temperature greater than 40 °C.[2]
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 (called anorexia), 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.[3] 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.[3] | 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 it is infected with bacteria, pain will occur 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.[3] A possibly infected 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.[4] 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.[5]
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 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 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:
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.
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 which can paralyze muscles, or staphylococcus releases toxins which can 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.[6] Chronic infections by parasites account for a high morbidity and mortality in many underdeveloped countries.
In order for infecting organisms to survive and repeat the cycle of infection in other hosts, they (or their progeny) must leave an existing reservoir and cause infection elsewhere. Transmission of infections 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 which have been contaminated by contact with an infecting organism is another case of disease transmission by indirect contact.[7]
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 pathogens which are transmitted by the fecal-oral route include Vibrio cholerae, Giardia species, rotaviruses, Entameba histolytica, Escherichia coli, and tape worms.[8] 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 [9]
H pylori is associated with inflammation of the stomach and is a common cause of stomach ulcers and gastritis. At least 10 percent of individuals infected with h pylori develop an ulcer. Moreover, there is an increased risk of stomach cancer after an infection with this organism.
Methicillin-resistant staphylococcus aureus predominantly affects the skin and is considered to be a super bug as it is very resistant to antibiotics. This bacteria is known to generate a variety of toxic enzymes which can lead to vomiting, diarrhea, shock and sepsis. MRSA is quite common in hospitals and today there is a great cause for concern about its spread.[10]
Chronic ear infections are a common problem in childhood. These infections may be due to bacteria or the common cold virus. The disorder often presents with persistent blockage of the ear, hearing loss, chronic ear drainage, balance problems, deep ear pain, headache, fever, excess sleepiness or confusion. Chronic ear infections usually develop slowly over many years in patients who have had ear problems. The treatment of persistent ear infections is complex and requires a combination of antibiotics, corticosteroids, and/or placement of tubes. When this fails surgery is required to control the infection.[11]
Osteomyelitis is a bone infection caused by various bacteria and can occur in both children and adults. When bone gets infected, there is continuous pain, fever and it is painful to move the extremity. Bone infections are acquired from an infection elsewhere in the body, trauma or spread from adjacent infected tissues. The diagnosis of bone infection requires a bone scan, blood cultures and x rays. Sometimes the bone marrow is aspirated to discover the specific organism. Osteomyelitis is a serious infection and carries a high complication rate if not treated promptly. If the infection is diagnosed rapidly, the prognosis is good. However chronic Osteomyelitis can take years to heal and can keep on recurring. Individuals at risk for Osteomyelitis include those who have artificial joints or metal components in the joint.[12]
Lyme is a tick borne disease that can cause a skin rash, fever, chills, body aches, and joint pain. Some individuals develop severe weakness and temporary paralysis. Lyme disease is caused by at least three species of bacteria belonging to the genus Borrelia, which are carried by deer ticks. One is more likely to get the infection during the summer months, especially if one spends time in grassy woodlands where ticks breed. When the infection is diagnosed promptly, most people do recover fully. However, there are some individuals who keep on having recurring or lingering symptoms long after the infection has been treated. When it becomes chronic, Lyme Disease can present with a variety of symptoms including migrating joint pains, headaches, confusion, excess fatigue, inability to sleep, paralysis of one side of the face, and difficulty concentrating. Even though there are reliable tests available they are not one hundred percent sensitive. While most individuals do respond to a 14 day course of antibiotics, some individuals take considerably longer.[13]
Chlamydia is a common sexually transmitted disease which can damage the female reproductive organs and result in permanent infertility if not treated promptly. Chlamydia is the most frequently contracted sexually transmitted bacterial infection in America. The organism is transmitted orally and through anal sex. Chlamydia is known as the silent disorder because many women who acquire this infection have no symptoms. Those who develop symptoms may complain of low back pain, painful intercourse, nausea, fever, bleeding, lower abdominal cramps, and a discharge. Chlamydia can be treated with antibiotics; however, women whose sex partners are not treated are at a very high risk for re-infection. Moreover, multiple infections increase the female's chances of sustaining damage to her reproductive organs, resulting in infertility. [14]
Many individuals develop a variety of infections in their lifetime, but quickly overcome them. However, some individuals develop chronic or persistent infections. In the majority of cases, persistent infections are caused by viruses and not bacteria. The common viruses that can cause chronic infection include: measles, hepatitis, herpes, infectious mononucleosis and Cytomegalovirus (CMV).[15] Bacteria can also cause chronic infections in individuals with diabetes,[16] those with compromised immunity, and in individuals who smoke.
The most common persistent infections in North America include: HIV, hepatitis, herpes simplex, and, common to all mammals, endogenous retroviruses, which play a crucial role in placentation[17] and therefore act as symbionts. Hepatitis B and C are usually acquired from the use of dirty needles, blood transfusions, or sexual intercourse. HIV has similar modes of transmission. Once hepatitis has been acquired, it becomes a chronic disorder. While some individuals with hepatitis B may remain asymptomatic, many will show active symptoms and remain infectious. In the long term, both hepatitis B and C can cause liver failure or liver cancer. In some cases, the signs and symptoms of liver damage may not appear for 20 years after the infection was initially acquired. Though not all viral, other persistent infections include: recurrent ear infection in children, tuberculosis, Lyme disease, Chlamydia, and malaria. The problem with recurrent infections is that the organism continues to damage the body which eventually results in symptoms. As the body weakens, the individual develops weight loss and extreme fatigue.
Viable treatment and prevention strategies will 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.[18] Depending on the severity and the type of infection, the antibiotic may be given by mouth, injection or may be applied topically.[19] 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.[20]
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.[21] Nutrition has to 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 eating foods which 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.[22] 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.[23]
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