Everything about Contagious Disease totally explained
An
infectious disease is a clinically evident
disease resulting from the presence of
pathogenic microbial agents, including
pathogenic viruses,
pathogenic bacteria,
fungi,
protozoa, multicellular
parasites, and aberrant proteins known as
prions. These
pathogens are able to cause disease in animals and/or plants.
Infectious pathologies are usually qualified as
contagious diseases (also called communicable diseases) due to their potentiality of transmission from one person or species to another. Transmission of an infectious disease may occur through one or more of diverse pathways including physical contact with infected individuals. These infecting agents may also be transmitted through liquids, food, body fluids, contaminated objects, airborne inhalation, or through
vector-borne spread. An
infection however, isn't
synonymous with an infectious disease, as an infection may not cause important clinical symptoms or impair host function.
Classification
Among the almost infinite varieties of microorganisms, relatively few cause disease in otherwise healthy individuals. Infectious disease results from the interplay between those few pathogens and the defenses of the hosts they infect. The appearance and severity of disease resulting from any pathogen depends upon the ability of that pathogen to damage the host as well as the ability of the host to resist the pathogen. Infectious microorganisms, or microbes, are therefore classified as either
primary pathogens or as
opportunistic pathogens according to the status of host defenses.
Primary pathogens cause disease as a result of their presence or activity within the normal, healthy host, and their intrinsic
virulence (the severity of the disease they cause) is, in part, a necessary consequence of their need to reproduce and spread. Many of the most common primary pathogens of humans only infect humans, however many serious diseases are caused by organisms acquired from the environment or which infect non-human hosts.
Organisms which cause an infectious disease in a host with depressed resistance are classified as
opportunistic pathogens. Opportunistic disease may be caused by microbes that are ordinarily in contact with the host, such as
pathogenic bacteria or fungi in the
gastrointestinal or the
upper respiratory tract, and they may also result from (otherwise innocuous) microbes acquired from other hosts (as in
Clostridium difficile colitis) or from the environment as a result of
traumatic introduction (as in
surgical wound infections or
compound fractures). An opportunistic disease requires impairment of host defenses, which may occur as a result of
genetic defects (such as
Chronic granulomatous disease), exposure to
antimicrobial drugs or
immunosuppressive chemicals (as might occur following
poisoning or
cancer chemotherapy), exposure to
ionizing radiation, or as a result of an infectious disease with immunosuppressive activity (such as with
measles,
malaria or
HIV disease). Primary pathogens may also cause more severe disease in a host with depressed resistance than would normally occur in an immunosufficient host.
One way of proving that a given disease is "infectious", is to satisfy
Koch's postulates (first proposed by
Robert Koch), which demands that the
infectious agent be identified only in patients and not in healthy controls, and that patients who contract the agent also develop the disease. These postulates were first used in the discovery that
Mycobacteria species cause
tuberculosis. Koch's postulates can't be met ethically for many human diseases because they require experimental infection of a healthy individual with a pathogen produced as a pure culture. Often, even diseases that are quite clearly infectious don't meet the infectious criteria. For example,
Treponema pallidum, the causative
spirochete of
syphilis, can't be
cultured in vitro - however the organism can be cultured in rabbit
testes. It is less clear that a pure culture comes from an animal source serving as host than it's when derived from microbes derived from plate culture.
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).
Transmission
An infectious disease is transmitted from some source. Defining the means of transmission plays an important part in understanding the biology of an infectious agent, and in addressing the disease it causes. Transmission may occur through several different mechanisms.
Respiratory diseases and
meningitis are commonly acquired by contact with aerosolized droplets, spread by sneezing, coughing, talking, kissing or even singing.
Gastrointestinal diseases are often acquired by ingesting contaminated food and water.
Sexually transmitted diseases are acquired through contact with bodily fluids, generally as a result of sexual activity. Some infectious agents may be spread as a result of contact with a contaminated, inanimate object (known as a
fomite), such as a coin passed from one person to another, while other diseases penetrate the
skin directly.
Transmission of infectious diseases may also involve a "
vector". Vectors may be mechanical or biological. A mechanical vector picks up an infectious agent on the outside of its body and transmits it in a passive manner. An example of a mechanical vector is a
housefly, which lands on cow dung, contaminating its appendages with bacteria from the feces, and then lands on food prior to consumption. The pathogen never enters the body of the fly. In contrast, biological vectors harbor pathogens within their bodies and deliver pathogens to new hosts in an active manner, usually a bite. Biological vectors are often responsible for serious
blood-borne diseases, such as
malaria,
viral encephalitis,
Chagas disease,
Lyme disease and
African sleeping sickness. Biological vectors are usually, though not exclusively,
arthropods, such as
mosquitoes,
ticks,
fleas and
lice. Vectors are often required in the life cycle of a pathogen. A common strategy, used to control vector borne infectious diseases, is to interrupt the life cycle of a pathogen, by killing the vector.
The relationship between virulence and transmission is complex, and has important consequences for the long term evolution of a pathogen. Since it takes many generations 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. 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's for instance in the case of cholera (the explosive diarrhea aids the bacterium in finding new hosts) or many respiratory infections (sneezing and coughing create infectious
aerosols).
Preventing transmission
One of the ways to prevent or slow down the transmission of infectious diseases is to recognize the different characteristics of various diseases. Some critical disease characteristics that should be evaluated include
virulence, distance traveled by victims, and level of contagiousness. The human strains of
Ebola virus, for example, incapacitate its victims extremely quickly and kills them soon after. As a result, the victims of this disease don't have the opportunity to travel very far from the initial infection zone. Also, this virus must spread through skin lesions or permeable membranes such as the eye. Thus, the initial stage of
Ebola isn't very contagious since its victims experience only internal hemorrhaging. As a result of the above features, the spread of
Ebola is very rapid and usually stays within a relatively confined geographical area. In contrast,
Human Immunodeficiency Virus (
HIV) kills its victims very slowly by attacking their immune system.
[6] As a result, a lot of its victims transmit the virus to many others before even realizing that they're carrying the disease. Also, the relatively low virulence allows its victims to travel long distances, increasing the likelihood of an
epidemic.
Another effective way to decrease the transmission rate of infectious diseases is to recognize the effects of
small-world networks.
[6] In
epidemics, there are often extensive interactions within hubs or groups of infected individuals and other interactions within discrete hubs of susceptible individuals. Despite the low interaction between discrete hubs, the disease can jump to and spread in a susceptible hub via a single or few interactions with an infected hub. Thus, infection rates in
small-world networks can be reduced somewhat if interactions between individuals within infected hubs are eliminated (Figure 1). However, infection rates can be drastically reduced if the main focus is on the prevention of transmission jumps between hubs. The use of needle exchange programs in areas with a high density of drug users with
HIV is an example of the successful implementation of this treatment method.
[6] Another example is the use of ring culling or vaccination of potentially susceptible livestock in adjacent farms to prevent the spread of the
foot-and-mouth virus in 2001.
Image:Small-WorldNetworks(CrossOuts).jpg| Figure 1: A simplified model of how disease transmission in small-world networks can be prevented. Major focus should be on preventing jumps between hubs (green cross out) in addition to prevention within infected hubs (red cross outs).
Diagnosis and therapy
Diagnosis of infectious disease sometimes involves identifying an infectious agent either directly or indirectly. In practice most minor infectious diseases such as
warts,
cutaneous abscesses,
respiratory system infections and
diarrheal diseases are diagnosed by their clinical presentation. Conclusions about the cause of the disease are based upon the likelihood that a patient came in contact with a particular agent, the presence of a microbe in a community, and other epidemiological considerations. Given sufficient effort, all known infectious agents can be specifically identified. The benefits of identification, however, are often greatly outweighed by the cost, as often there's no specific treatment, the cause is obvious, or the outcome of an infection is
benign.
Specific identification of an infectious agent is usually only determined 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 wouldn't 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. 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's 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.
Methods of diagnosis
Diagnosis of infectious disease is nearly always initiated by medical history and physical examination. More detailed identification techniques involve the culture of infectious agents isolated from a patient. Culture allows identification of infectious organisms by examining their microscopic features, by detecting the presence of substances produced by pathogens, and by directly identifying an organism by its genotype. Other techniques (such as
X-rays,
CAT scans,
PET scans or
NMR) are used to produce images of internal abnormalities resulting from the growth of an infectious agent. The images are useful in detection of, for example, a bone
abscess or a
spongiform encephalopathy produced by a
prion.
Microbial culture
Microbiological culture is a principal tool used to diagnose infectious disease. In a microbial culture, a
growth medium is provided for a specific agent. A sample taken from potentially diseased tissue or fluid is then tested for the presence of an infectious agent able to grow within that medium. Most pathogenic bacteria are easily grown on nutrient
agar, a form of solid medium that supplies carbohydrates and proteins necessary for growth of a bacterium, along with copious amounts of water. A single bacterium will grow into a visible mound on the surface of the plate called a
colony, which may be separated from other colonies or melded together into a "lawn". The size, color, shape and form of a colony is characteristic of the bacterial species, its specific genetic makeup (its strain), and the environment which supports its growth. Other ingredients are often added to the plate to aid in identification. Plates may contain substances that permit the growth of some bacteria and not others, or that change color in response to certain bacteria and not others. Bacteriological plates such as these are commonly used in the clinical identification of infectious
bacteria. Microbial culture may also be used in the identification of
viruses: the medium in this case being cells grown in culture that the virus can infect, and then alter or kill. In the case of viral identification, a region of dead cells results from viral growth, and is called a "plaque".
Eukaryotic parasites may also be grown in culture as a means of identifying a particular agent.
In the absence of suitable plate culture techniques, some microbes require culture within live animals. Bacteria such as
Mycobacterium leprae and
T. pallidum can be grown in animals, although serological and microscopic techniques make the use of live animals unnecessary. Viruses are also usually identified using alternatives to growth in culture or animals. Some viruses may be grown in
embryonated eggs. Another useful identification method is
Xenodiagnosis, or the use of a vector to support the growth of an infectious agent.
Chaga's disease is the most significant example, because it's difficult to directly demonstrate the presence of the causative agent,
Trypanosoma cruzi in a patient, which therefore makes it difficult to definitively make a diagnosis. In this case, xenodiagnosis involves the use of the
vector of the Chaga's agent
T. cruzi, an uninfected triatomine bug (subfamily
Triatominae), which takes a blood meal from a person suspected of having been infected. The bug is later inspected for growth of
T. cruzi within its gut.
Microscopy
Another principle tool in the diagnosis of infectious disease is
microscopy. Virtually all of the culture techniques discussed above rely, at some point, on microscopic examination for definitive identification of the infectious agent.
Microscopy may be carried out with simple instruments, such as the compound
light microscope, or with instruments as complex as an
electron microscope. Samples obtained from patients may be viewed directly under the light microscope, and can often rapidly lead to identification. Microscopy is often also used in conjunction with
biochemical staining techniques, and can be made exquisitely specific when used in combination with
antibody based techniques. For example, the use of
antibodies made artificially
fluorescent (fluorescently labeled antibodies) can be directed to bind to and identify a specific
antigens present on a pathogen. A
fluorescence microscope is then used to detect fluorescently labeled antibodies bound to internalized antigens within clinical samples or cultured cells. This technique is especially useful in the diagnosis of viral diseases, where the light microscope is incapable of identifying a virus directly.
Other microscopic procedures may also aid in identifying infectious agents. Almost all cells readily stain with a number of basic
dyes due to the
electrostatic attraction between negatively charged cellular molecules and the positive charge on the dye. A cell is normally transparent under a microscope, and using a stain increases the contrast of a cell with its background. Staining a cell with a dye such as
Giemsa stain or
crystal violet allows a microscopist to describe its size, shape, internal and external components and its associations with other cells. The response of bacteria to different staining procedures is used in the
taxonomic classification of microbes as well. Two methods, the
Gram stain and the
acid-fast stain, are the standard approaches used to classify bacteria and to diagnosis of disease. The Gram stain identifies the bacterial groups
Firmicutes and
Actinobacteria, both of which contain many significant human pathogens. The acid-fast staining procedure identifies the Actinobacterial genera
Mycobacterium and
Nocardia.
Biochemical tests
Biochemical tests used in the identification of infectious agents include the detection of
metabolic or
enzymatic products characteristic of a particular infectious agent. Since bacteria ferment
carbohydrates in patterns characteristic of their
genus and
species, the detection of
fermentation products is commonly used in bacterial identification.
Acids,
alcohols and
gases are usually detected in these tests when bacteria are grown in
selective liquid or solid media.
The isolation of
enzymes from infected tissue can also provide the basis of a biochemical diagnosis of an infectious disease. For example, humans can make neither
RNA replicases nor
reverse transcriptase, and the presence of these enzymes are characteristic of specific types of viral infections. The ability of the viral protein
hemagglutinin to bind
red blood cells together into a detectable matrix may also be characterized as a biochemical test for viral infection, although strictly speaking hemagglutinin isn't an
enzyme and has no metabolic function.
Serological methods are highly sensitive, specific and often extremely rapid tests used to identify microorganisms. These tests are based upon the ability of an antibody to bind specifically to an antigen. The antigen, usually a protein or carbohydrate made by an infectious agent, is bound by the antibody. This binding then sets off a chain of events that can be visibly obvious in various ways, dependent upon the test. For example, "
Strep throat" is often diagnosed within minutes, and is based on the appearance of antigens made by the causative agent,
S. pyogenes, that's retrieved from a patients throat with a cotton swab. Serological tests, if available, are usually the preferred route of identification, however the tests are costly to develop and the reagents used in the test often require
refrigeration. Some serological methods are extremely costly, although when commonly used, such as with the "strep test", they can be inexpensive.
Molecular diagnostics
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're 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 won't 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 doesn't occur, this limits the ability of PCR to detect the presence of any bacteria.
Clearance and immunity
Infection with most pathogens doesn't result in death of the host and the offending organism is ultimately cleared after the symptoms of the disease have waned.
! Rank|| Cause of death || Deaths 2002 || Percentage of
all deaths || Deaths 1993|| 1993 Rank
|-
|colspan="1" style="background:#ffdead;"|N/A||colspan="1" style="background:#ffdead;" | All infectious diseases ||colspan="1" style="background:#ffdead;" |14.7 million ||colspan="1" style="background:#ffdead;" align="center"|25.9% ||colspan="1" style="background:#ffdead;"| 16.4 million || colspan="1" style="background:#ffdead;" align="center" |32.2%
|-
| 1 ||
Lower respiratory infections
||3.9 million ||align="center"|6.9%||4.1 million||1
|-
| 2 |||
HIV/
AIDS ||2.8 million || align="center" |4.9% || 0.7 million||7
|-
| 3 ||
Diarrheal diseases||1.8 million ||align="center" |3.2% ||3.0 million||2
|-
| 4 ||
Tuberculosis (TB)|| 1.6 million || align="center" |2.7% ||2.7 million ||3
|-
| 5 ||
Malaria ||1.3 million ||align="center" |2.2% ||2.0 million ||4
|-
| 6 ||
Measles ||0.6 million ||align="center" |1.1% ||1.1 million ||5
|-
| 7 ||
Pertussis ||0.29 million||align="center" |0.5% ||0.36 million ||7
|-
| 8 ||
Tetanus ||0.21 million ||align="center" |0.4% ||0.15 million ||12
|-
| 9 ||
Meningitis||0.17 million ||align="center" |0.3% ||0.25 million ||8
|-
| 10 ||
Syphilis||0.16 million ||align="center" |0.3% ||0.19 million || 11
|-
| 11 ||
Hepatitis B ||0.10 million ||align="center" |0.2% ||0.93 million || 6
|-
| 12-17 ||
Tropical diseases (6) ||0.13 million ||align="center" |0.2% ||0.53 million ||9, 10, 16-18
|-
|align="left" colspan="6"|
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
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 percent of Europe's population.
The Black Death of 1347 to 1352 killed 25 million in Europe over 5 years (estimated to be between 25 and 50% of the populations of Europe, Asia, and Africa - the world population at the time was 500 million).
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.
The first European influenza epidemic occurred between 1556 and 1560, with an estimated mortality rate of 20%.
The Influenza Pandemic of 1918 (or the Spanish Flu) killed 25-50 million people (about 2% of world population of 1.7 billion). Today Influenza kills about 250,000 to 500,000 worldwide each year.
Emerging diseases and pandemics
In most cases, microorganisms live in harmony with their hosts. Such is the case for many tropical viruses and the insects, monkeys, or other animals in which they've lived and reproduced. Because the microbes and their hosts have co-evolved, the hosts gradually become resistant to the microorganisms. When a microbe jumps from a long-time animal host to a human being, it may cease to be a harmless parasite and become pathogenic.
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 niche. When 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 to cause human disease.
Several human activities have led to the emergence and spread of new diseases, He also stated that bodily secretion is contaminated by foul foreign earthly bodies before being infected, but he didn't view them as primary causes of disease.
When the Black Death bubonic plague reached al-Andalus in the 14th century, Ibn Khatima and Ibn al-Khatib hypothesized that infectious diseases are caused by "contagious entities" which enter the human body.
Anton van Leeuwenhoek (1632-1723) advanced the science of microscopy by being the first to observe microorganisms, allowing for easy visualization of bacteria.
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.
Gerhard Domagk develops 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 hasn't 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 (for example tropical diseases);
The disease hasn't responded to first line antibiotics;
The disease might be dangerous to other patients, and the patient might have to be isolatedFurther Information
Get more info on 'Contagious Disease'.
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