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Basics Infectious diseases are the invasion of a Host organism by a foreign Replicator , generally microorganisms, often called ''microbes,'' that are invisible to the naked eye. Microbes that cause illness are also known as ''pathogens.'' The most common pathogens are various bacteria and viruses, though a number of other microorganisms, including some kinds of Fungi and Protozoa, also cause disease. Prions are borderline, and Meme s would not usually be considered in this scope. An infectious disease is termed ''contagious'' if it is easily transmitted from one person to another. An organism that a microbe infects is known as the host for that microbe. In the human host, a microorganism causes disease by either disrupting a vital body process or stimulating the immune system to mount a defensive reaction. An immune response against a pathogen, which can include a high fever, inflammation, and other damaging symptoms, can be more devastating than the direct damage caused by the microbe. Agents and vectors Infectious disease requires an ''agent'' and a ''mode of transmission'' (or '' Vector ''). A good example is Malaria , which is caused by Plasmodial Parasite s, chiefly '' Plasmodium Falciparum '' but does not affect humans unless the vector, the Anopheles Mosquito , is around to introduce the parasite into the Human bloodstream. The vector does not have to be biological. Many infectious diseases are transmitted by droplets which enter the airway (e.g. Common Cold and Tuberculosis ). Mortality from infectious diseases 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 also included for comparison. (Note: The other categories of death are 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 . Lower respiratory infections (which include various Pneumonia s) and diarrheal diseases are caused by many different Virus es, Bacteria or Parasite s. Childhood diseases include Pertussis , Poliomyelitis , Diphtheria , Measles , and Tetanus . Children also make up a large percentage of lower respiratory and diarrheal deaths. Tropical diseases include Trypanosomiasis , Chagas Disease . Schistosomiasis , Leishmaniasis , Lymphatic Filariasis , and Onchocerciasis . From 1993 to 2002, the death ranking for AIDS went up from 7th to 2nd and Hepatitis B went down from 6th to 11th. While the number of deaths has decreased in almost every disease, it has increased four-fold in AIDS . Historic pandemics A Pandemic (or global Epidemic ) is a disease that affects people over an extensive geographical area.
New diseases and pandemics In some cases, a microorganism and its host live in reasonable harmony. Such is the case for many tropical viruses and the insects, monkeys, or other animals in which they have lived and reproduced for thousands or millions of years. Because the microbes and their hosts have co-evolved together, the hosts have gradually become resistant to the microorganisms. But when a microbe jumps from a long-time animal host to a human being, it may cease being a harmless parasite and—simply because it is new to the human species—become a pathogen. (See Infection ). With most new infectious diseases, some human action is involved, changing the environment so that an existing Microbe can take up residence in a new ecological Niche. Once that happens, a Pathogen that had been confined to a remote habitat appears in a new or wider region, or a microbe that had infected only animals suddenly begins causing human disease. Several human activities have led to the emergence and spread of new diseases: :Encroachment on wildlife habitats. The construction of new villages and housing developments in rural areas brings people into contact with animals--and the microbes they harbor. :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. :Destroying rain forests. As tropical countries make use of their rain forests, building roads through forests and clearing areas for settlement or commercial ventures, people encounter insects and other animals harboring unknown microorganisms. :Uncontrolled urbanization. The rapid growth of cities in many developing countries concentrates large numbers of people in crowded areas with poor sanitation, which foster the transmission of contagious diseases. :Modern transport. Ships and other cargo carriers often harbor unintended "passengers," such as insects and rats, that can spread diseases to faraway destinations. :High-speed globe-trotting. With international jet-airplane travel, people infected with a new disease can carry the disease to the far side of the world before their first symptoms appear. The relationship between virulence and transmission is complex, and has important consequences for the long term evolution of a pathogen. If a disease is rapidly fatal, the host may die before the microbe can get passed along to another host. However, this cost may be overwhelmed by the short term benefit of higher infectiousness if transmission is linked to virulence, as it is for instance in the case of cholera (the explosive diarrhoea aids the bacterium in finding new hosts) or many respiratory infections (sneezing, coughing etc create infectious aerosols). Since it takes time for a microbe and a new host species to co-evolve an emerging pathogen may hit its earliest victims especially hard. It is usually in the first wave of a new disease that death rates are highest. Diagnosis and therapy The field of ''infectious diseases'' also occupies itself with the Diagnosis and Therapy of Infection . Diagnosis ''Diagnosis'' is initially by medical history and physical examination, and imaging (such as X-ray s), but the principal tool in infectious disease is the Microbiological Culture . In a culture, a Growth Medium is provided for a particular agent. After inoculation of a specimen of diseased fluid or tissue onto the medium, it is determined whether bacterial growth occurs. This works for a number of Bacteria , for example Staphylococcus or Streptococcus . Certain agents cannot be Cultured , for example the above-mentioned '' Treponema Pallidum '' and most Virus es. The first Serological markers were developed to diagnose Syphilis (the Wassermann Test , later replaced by the VDRL and TPHA tests). Serology involves detecting the Antibodies against an infectious agent in the patient's Blood . In Immunocompromised patients (e.g. AIDS ), Serology can be troublesome, because the antibody reaction is blunted. A more recent development is direct detection of Viral proteins and/or DNA in Blood or Secretion s. This can be done by PCR (polymerase chain reaction), involving the amplification of viral DNA and its subsequent detection with anti-DNA probes. The classification of infectious disease One way of proving that a given disease is "infectious", is to satisfy Koch's Postulates ( Robert Koch ), which demand that the Infectious Agent is identified in patients and not in controls, and that patients who contract the agent also develop the disease. These postulates were tried and tested in the discovery of Mycobacteria as the cause for Tuberculosis . Often, it is not possible to meet some of the criteria, even in diseases that are quite clearly infectious. For example, '' Treponema Pallidum '', the causative Spirochete of Syphilis , cannot be Cultured in vitro - however the organism can be cultured in rabbit testes]. Epidemiology is another important tool used to study disease in a population. For infectious diseases it helps to determine if a disease Outbreak is sporadic (occasional occurrence), Endemic (regular cases often occurring in a region), Epidemic (an unusually high number of cases in a region), or Pandemic (a global epidemic). Therapy When a '' can then be used to determine optimal therapy for the patient. This can reduce the use of Broad-spectrum Antibiotic s and lead to a decrease in Antibiotic Resistance . The work of an infectiologist Doctor s who specialise in the Medical Treatment of infectious disease are called ''infectiologists'' or ''infectious disease specialists''. Generally, Infection s 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 services of the infectious disease team are called for when:
The work of the infectiologist therefore entails working with patients and doctors on one hand and Laboratory Scientists and Immunologist s on the other hand. History Anton Van Leeuwenhoek ( 1632 - 1723 ) advanced the science of microscopy, allowing easy visualization of bacteria. Louis Pasteur proved beyond doubt that certain diseases can be caused by infectious agents, and developed a vaccine for rabies. Robert Koch , mentioned above, gave the study of infectious diseases a scientific basis by formulating Koch's Postulates . Edward Jenner , Jonas Salk and Albert Sabin developed successful vaccines for Smallpox and Polio , reducing the threat of these debilitating diseases. See also
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