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Technologies based upon the polymerase chain reaction (PCR) method will become nearly ubiquitous gold standards of diagnostics of the near future, for several reasons. First, the catalog of infectious agents has grown to the point that virtually all of the significant infectious agents of the human population have been identified. Second, an infectious agent must grow within the human body to cause disease; essentially it must amplify its own nucleic acids in order to cause a disease. This amplification of nucleic acid in infected tissue offers an opportunity to detect the infectious agent by using PCR. Third, the essential tools for directing PCR, primers, are derived from the genomes of infectious agents, and with time those genomes will be known, if they are not already. Thus, the technological ability to detect any infectious agent rapidly and specifically are currently available. The only remaining blockades to the use of PCR as a standard tool of diagnosis are in its cost and application, neither of which is insurmountable. The diagnosis of a few diseases will not benefit from the development of PCR methods, such as some of the clostridial diseases (tetanus and botulism). These diseases are fundamentally biological poisonings by relatively small numbers of infectious bacteria that produce extremely potent neurotoxins. A significant proliferation of the infectious agent does not occur, this limits the ability of PCR to detect the presence of any bacteria. Indication of tests There is usually an indication for a specific identification of an infectious agent only when such identification can aid in the treatment or prevention of the disease, or to advance knowledge of the course of an illness prior to the development of effective therapeutic or preventative measures. For example, in the early 1980s, prior to the appearance of AZT for the treatment of AIDS, the course of the disease was closely followed by monitoring the composition of patient blood samples, even though the outcome would not offer the patient any further treatment options. In part, these studies on the appearance of HIV in specific communities permitted the advancement of hypotheses as to the route of transmission of the virus. By understanding how the disease was transmitted, resources could be targeted to the communities at greatest risk in campaigns aimed at reducing the number of new infections. The specific serological diagnostic identification, and later genotypic or molecular identification, of HIV also enabled the development of hypotheses as to the temporal and geographical origins of the virus, as well as a myriad of other hypothesis.[5] The development of molecular diagnostic tools have enabled physicians and researchers to monitor the efficacy of treatment with anti-retroviral drugs. Molecular diagnostics are now commonly used to identify HIV in healthy people long before the onset of illness and have been used to demonstrate the existence of people who are genetically resistant to HIV infection. Thus, while there still is no cure for AIDS, there is great therapeutic and predictive benefit to identifying the virus and monitoring the virus levels within the blood of infected individuals, both for the patient and for the community at large. Anti-infective treatments [icon] This section requires expansion. (June 2014) This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (June 2014) 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.[23][better source needed] Depending on the severity and the type of infection, the antibiotic may be given by mouth, injection or may be applied topically.[24][bare URL] 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.[25] Epidemiology Disability-adjusted life year for infectious and parasitic diseases per 100,000 inhabitants in 2004.[26] no data ≤250 250-500 500-1000 1000-2000 2000-3000 3000-4000 4000-5000 5000-6250 6250-12500 12500-25000 25000-50000 ≥50000 The World Health Organization collects information on global deaths by International Classification of Disease (ICD) code categories. The following table lists the top infectious disease killers which caused more than 100,000 deaths in 2002 (estimated). 1993 data is included for comparison. Worldwide mortality due to infectious diseases[27][28] Rank Cause of death Deaths 2002 (in millions) Percentage of all deaths Deaths 1993 (in millions) 1993 Rank N/A All infectious diseases 14.7 25.9% 16.4 32.2% 1 Lower respiratory infections[29] 3.9 6.9% 4.1 1 2 HIV/AIDS 2.8 4.9% 0.7 7 3 Diarrheal diseases[30] 1.8 3.2% 3.0 2 4 Tuberculosis (TB) 1.6 2.7% 2.7 3 5 Malaria 1.3 2.2% 2.0 4 6 Measles 0.6 1.1% 1.1 5 7 Pertussis 0.29 0.5% 0.36 7 8 Tetanus 0.21 0.4% 0.15 12 9 Meningitis 0.17 0.3% 0.25 8 10 Syphilis 0.16 0.3% 0.19 11 11 Hepatitis B 0.10 0.2% 0.93 6 12-17 Tropical diseases (6)[31] 0.13 0.2% 0.53 9, 10, 16-18 Note: Other causes of death include maternal and perinatal conditions (5.2%), nutritional deficiencies (0.9%), noncommunicable conditions (58.8%), and injuries (9.1%). The top three single agent/disease killers are HIV/AIDS, TB and malaria. While the number of deaths due to nearly every disease have decreased, deaths due to HIV/AIDS have increased fourfold. Childhood diseases include pertussis, poliomyelitis, diphtheria, measles and tetanus. Children also make up a large percentage of lower respiratory and diarrheal deaths. Historic pandemics Great Plague of Marseille in 1720 killed 100,000 people in the city and the surrounding provinces A pandemic (or global epidemic) is a disease that affects people over an extensive geographical area. Plague of Justinian, from 541 to 750, killed between 50% and 60% of Europe's population.[32] The Black Death of 1347 to 1352 killed 25 million in Europe over 5 years. The plague reduced the world population from an estimated 450 million to between 350 and 375 million in the 14th century. The introduction of smallpox, measles, and typhus to the areas of Central and South America by European explorers during the 15th and 16th centuries caused pandemics among the native inhabitants. Between 1518 and 1568 disease pandemics are said to have caused the population of Mexico to fall from 20 million to 3 million.[33] The first European influenza epidemic occurred between 1556 and 1560, with an estimated mortality rate of 20%.[33] Smallpox killed an estimated 60 million Europeans during the 18th century[34] (approximately 400,000 per year).[35] Up to 30% of those infected, including 80% of the children under 5 years of age, died from the disease, and one-third of the survivors went blind.[36] In the 19th century, tuberculosis killed an estimated one-quarter of the adult population of Europe;[37] by 1918 one in six deaths in France were still caused by TB. The Influenza Pandemic of 1918 (or the Spanish Flu) killed 25-50 million people (about 2% of world population of 1.7 billion).[38] Today Influenza kills about 250,000 to 500,000 worldwide each year. Emerging diseases In most cases, microorganisms live in harmony with their hosts via mutual or commensal interactions. Diseases can emerge when existing parasites become pathogenic or when new pathogenic parasites enter a new host. Coevolution between parasite and host can lead to hosts becoming resistant to the parasites or the parasites may evolve greater virulence, leading to immunopathological disease. Human activity is involved with many emerging infectious diseases, such as environmental change enabling a parasite to occupy new niches. When that happens, a pathogen that had been confined to a remote habitat has a wider distribution and possibly a new host organism. Parasites jumping from nonhuman to human hosts are known as zoonoses. Under disease invasion, when a parasite invades a new host species, it may become pathogenic in the new host.[39] Several human activities have led to the emergence of zoonotic human pathogens, including viruses, bacteria, protozoa, and rickettsia,[40] and spread of vector-borne diseases,[39] see also Globalization and Disease and Wildlife disease: Encroachment on wildlife habitats. The construction of new villages and housing developments in rural areas force animals to live in dense populations, creating opportunities for microbes to mutate and emerge.[41] Changes in agriculture. The introduction of new crops attracts new crop pests and the microbes they carry to farming communities, exposing people to unfamiliar diseases. The destruction of rain forests. As countries make use of their rain forests, by building roads through forests and clearing areas for settlement or commercial ventures, people encounter insects and other animals harboring previously unknown microorganisms. Uncontrolled urbanization. The rapid growth of cities in many developing countries tends to concentrate large numbers of people into crowded areas with poor sanitation. These conditions foster transmission of contagious diseases. Modern transport. Ships and other cargo carriers often harbor unintended "passengers", that can spread diseases to faraway destinations. While with international jet-airplane travel, people infected with a disease can carry it to distant lands, or home to their families, before their first symptoms appear. History East German postage stamps depicting four antique microscopes. Advancements in microscopy were essential to the early study of infectious diseases. Ideas of contagion became more popular in Europe during the Renaissance, particularly through the writing of the Italian physician Girolamo Fracastoro.[42] Anton van Leeuwenhoek (1632–1723) advanced the science of microscopy by being the first to observe microorganisms, allowing for easy visualization of bacteria. In the mid-19th century John Snow and William Budd did important work demonstrating the contagiousness of typhoid and cholera through contaminated water. Both are credited with decreasing epidemics of cholera in their towns by implementing measures to prevent contamination of water.[43] Louis Pasteur proved beyond doubt that certain diseases are caused by infectious agents, and developed a vaccine for rabies. Robert Koch, provided the study of infectious diseases with a scientific basis known as Koch's postulates. Edward Jenner, Jonas Salk and Albert Sabin developed effective vaccines for smallpox and polio, which would later result in the eradication and near-eradication of these diseases, respectively. Alexander Fleming discovered the world's first antibiotic Penicillin which Florey and Chain then developed. Gerhard Domagk developed sulphonamides, the first broad spectrum synthetic antibacterial drugs. Medical specialists The medical treatment of infectious diseases falls into the medical field of Infectiology and in some cases the study of propagation pertains to the field of Epidemiology. Generally, infections are initially diagnosed by primary care physicians or internal medicine specialists. For example, an "uncomplicated" pneumonia will generally be treated by the internist or the pulmonologist (lung physician).The work of the infectiologist therefore entails working with both patients and general practitioners, as well as laboratory scientists, immunologists, bacteriologists and other specialists. An infectious disease team may be alerted when: The disease has not been definitively diagnosed after an initial workup The patient is immunocompromised (for example, in AIDS or after chemotherapy); The infectious agent is of an uncommon nature (e.g. tropical diseases); The disease has not responded to first line antibiotics; The disease might be dangerous to other patients, and the patient might have to be isolated Society and culture A number of studies have reported associations between pathogen load in an area and human behavior. Higher pathogen load is associated with decreased size of ethnic and religious groups in an area. This may be due high pathogen load favoring avoidance other groups which may reduce pathogen transmission or a high pathogen load preventing the creation of large settlements and armies which enforce a common culture. Higher pathogen load is also associated with more restricted sexual behavior which may reduce pathogen transmission. It also associated with higher preferences for health and attractiveness in mates. Higher fertility rates and shorter or less parental care per child is another association which may be a compensation for the higher mortality rate. There is also an association with polygyny which may be due to higher pathogen load making selecting males with a high genetic resistance increasingly important. Higher pathogen load is also associated with more collectivism and less individualism which may limit contacts with outside groups and infections. There are alternative explanations for at least some of the associations although some of these explanations may in turn ultimately be due to pathogen load. Thus, polygny may also be due to a lower male:female ratio in these areas but this may ultimately be due to male infants having increased mortality from infectious diseases. Another example is that poor socioeconomic factors may ultimately in part be due to high pathogen load preventing economic development.[44] Fossil record Main article: Paleopathology Skull of dinosaur with long jaws and teeth. Herrerasaurus skull. 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.[45] |
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