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 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. "When infection attacks the body, anti-infective drugs can help turn the tide of battle. Four types of anti-infective drugs exist: antibacterial, antiviral, antitubercular, and antifungal.
Many individuals develop a variety of infections but quickly overcome them. However, some individuals are unfortunate and 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, various prion infections that affect the brain (mad cow) , herpes, infectious mononucleosis and Cytomegalovirus (CMV). Bacteria can also cause chronic infections in individuals with diabetes, those with compromised immunity and in individuals who smoke.
The diagnosis of persistent infections can be difficult as there are no specific signs and symptoms. If an infection is suspected, blood, urine and sputum cultures are usually the first step. Chest x ray and stool analysis may provide a clue. Sometimes fluid from the spinal cord is obtained 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, clinician instinct, and temperature greater than 40 C.
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.
Bacterial and viral infections can both cause symptoms such as malaise, fever, and chills. 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.
|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 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|
For infection to occur in a human, the chain of events must happen. Infections do not occur by chance but involve many factors. The chain of events involves several steps which include the infectious agent, reservoir, susceptible host, portal of entry, mode of transmission and portal of exit. 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. Each one of these links has an integral role in infection 
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.
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. Another word for an infection with no symptoms is "asymptomatic".
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 resulting in recurrent relapses. 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 the right circumstances arise.
Throughout the globe, persistent infections claim millions of lives each year  Chronic infections by parasites account for a high morbidity and mortality in many underdeveloped countries.
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:
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.
People should understand that infection and disease is not the same thing. Infection occurs when an organism enters the body and starts to grow. However, disease only occurs if the organism starts to multiply and produce symptoms. Our bodies have tremendous capacity to fight off organisms. Diseases result when these protective mechanism fail or are compromised.
Moreover not all infectious agents are likely to cause disease. For example the polio virus infects many people, but in less than 5% of individuals does it actually cause disease. On the other hand, some organisms are not contagious but very virulent. The mad cow virus kills almost everyone who is infected. The most dangerous organisms are those that are both very virulent and contagious . All organisms must enter the body in order to cause disease. The organism must stick or adhere to a specific cell, invade, colonize and inflict some type of damage to the host. This chain of events is the same for all organisms
Entrance to the host generally occurs through the normal openings like the oral cavity, nose, eyes, genitalia, anus, or open wounds. While a few organisms can grow at the initial site of entry, many invade and start to grow in different organs where they are hard to detect. Some organisms grow within the host cells whereas others grow freely in blood. Micro organisms 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.
For an organism to cause disease, there are several factors that must be met before an infection can occur. The organism must be able to grow, multiply, be able to enter the body and have the ability to cause disease. Infectious agents which cause disease in humans include bacteria, viruses, parasites and fungi.
Reservoir is a place where organisms can thrive and multiply. This may be either inside the human body or outside like in an inanimate environment. For example some organisms may thrive in water (mycobacterium tuberculosis), AC units (legionella), furniture (virus) or in a tick (Lyme disease). The reservoir for a disease is the site where the infectious agent survives. Humans are reservoir for the measles virus because this organism does not infect any other mammals. Beside insects, animals also serve as reservoirs for infectious organism. The rabies virus is carried in many small mammals and wild dogs.
In order for micro organism to survive and repeat the cycle of infection in other humans, it must have a way to exit the reservoir. For example, if the organism was growing in mouldy water in an air conditioning unit it may spread when the unit is switched on. In other cases, the organism may spread from humans to others when an infected individual coughs. Micro-organisms can also be passed from body fluids, faecal material and urine.
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 the right 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.
A common method of transmission in under developed countries is faecal oral transmission. In such cases, sewage water is used to wash food or is consumed. This results in food poisoning. Common disorders which are transmitted by the faecal oral route include cholera, giardia, rotavirus, cryptosporidium, ecoli or tape worms 
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 
Organisms need a point of entry. Some enter via the mucus membrane like the mouth, vagina or nose. Others enter via breaks in the skin, for example a surgical incision or a laceration. Sometimes physicians insert tubes or catheters into the bladder which can cause urinary tract infections. Even an intravenous line can become infected at the site where the needle hole was made on the skin.
Most humans are not easily infected. Organisms usually cause infections in people who are weak, sick, malnourished, have cancer, are diabetic or are immuno-suppressed. Individuals who have a suppressed immune system are quickly over powered by the organisms. The majority of chronic or persistent infections occur in individuals who have poor defense mechanism(s).
The most common persistent infections in North America include HIV, hepatitis and herpes simplex. Hepatitis B and C are usually acquired from 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 induce liver cancer. In some cases, the signs and symptoms of liver damage may not appear for 20 years after the infection was initially acquired. 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 body which eventually results in symptoms. As the body weakens, the individual develops weight loss and extreme fatigue.
Hepatitis B is spread via contact with blood, body fluids, use of infected needles, having sex with an infected individual or transmitted during childbirth. Once acquired, this form of hepatitis causes liver damage and a lifelong infection. It is highly recommended that one get vaccinated against the hepatitis B virus.
Chronic ear infections are a common problem in childhood. These infections may be due to a 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.
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 has to be aspirated to discover the right 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.
Lyme is a tick borne disease that can cause a skin rash, fever, chills, body aches and joint pain. Some unlucky individuals even develop severe weakness and temporary paralysis. Lyme disease is caused by the deer tick. One is more likely to get the infection during the summer months, especially if one spends time in grassy woodlands where the tick breeds. 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. Lyme disease when it becomes chronic 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 a lot longer.
Chlamydia is a common sexually transmitted disease which can damage the female sex reproductive organs if not treated promptly. Chlamydia can silently damage the reproductive organs which can result in permanent infertility. Chlamydia is the most frequently transmitted sexually disease in America. The organism is also transmitted orally and anal sex. Chlamydia is also 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. When diagnosed Chlamydia can be treated with antibiotics. However, women whose sex partners are not treated are at a very high risk for re infection. More over multiple infections increase the female's chances of damaging their reproductive organs resulting in infertility 
Appreciation of the infectious cycle is vital in order to recognize available targets for treatment strategies The infection cycle may be broken up and disease may be prevented from developing at different spots along the cycle. For example, direct people to people transmission can be diminished by adequate hygiene, maintaining a sanitary environment as well as health education. Other means of preventing an infection by an organism may be from vaccination.
Bacterial infections are usually treated with antibiotics. There are many types of antibiotics. 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.
Knowing the chain of infection is important for prevention. For examples techniques like hand washing, gowning and wearing face masks can help prevent infections from the surgeon to the patient or vice versa. Frequent hand washing remains the most important factors in preventing spread of non wanted organisms. Nutrition has to be improved and one has to make changes in life style- like avoiding 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.
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Some diseases are caused by infection. Infection is when another organism gets into a person's body and causes harm. There are many kinds of infectious agents: viruses, bacteria, parasites, and fungus. These four kinds of infectious agents cause most infectious disease. There are other kinds though. For example mad cow disease (Bovine spongiform encephalopathy) is caused by prions. Prions are a form of virus that are basically abnormal clumps of proteins. We are not even sure that prions are alive. But they act just like other infectious diseases.
Some infections are caused by different infectious agents at different times. For example, some diarrhea is caused by bacteria. Some is caused by viruses.
This is a very short list. There are many more diseases from infection and other causes: common diseases.
Some diseases can be passed from one person to another easily. Other infectious diseases are harder to get. If a person kiss or touches another person who is infected with the flu, a cold, measles, or a sore throat, he or she may get their disease. They may also give the person their disease if they cough on them. Other diseases like AIDS, herpes, and hepatitis B, are only passed by closer contact. Having sex with an infected person, blood transfusions, or touching their blood or urine can sometimes pass these diseases.
People can stop disease by:
Bacteria are usually treated with antibiotic medicines like Tetracycline and Penicillin. Viruses will not be killed by antibiotics. They must be treated with special medicines called antivirals. Medicines to treat HIV/AIDS like zidovudine are antivirals. Antifungals like miconazole and fluconazole treat infections from fungus. Anti-parasitic medicines like Praziquantel and mebendazole.
Some infectious diseases have no treatments that work well. For example, there is no medicine that works well to cure West Nile Virus and Rabies. So it is important to avoid getting these diseases.
Some infectious disease goes away on its own. These mild diseases do not need to be treated. For example colds are caused by viruses and they do not need medicines.
Some diseases can be prevented by immunizations. Immunizations try to make a person have an immune reaction to an infectious agent. This is usually done before the person gets the disease. Examples of diseases prevented by immunizations are: polio, tetanus, and pertussis. In some cases, if all of the disease can be removed from society, we can get rid of the disease completely. An example of this is smallpox. This virus used to kill people, cause blindness, and bad scars. Now there has not been a person who had smallpox in over thirty years.