Infectious diseases

In medicine, infectious disease or communicable disease is diseasecaused by a biologicalagent such as by a virus, bacteriumor parasite. This is contrasted to physicalcauses, such as burnsor chemical ones such as through intoxication.

Basics

Infectious diseases are the invasion of a hostorganism 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 fungiand protozoa,also cause disease. Prionsare borderline, and memeswould 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 parasites, chiefly Plasmodium falciparum but does not affect humans unless the vector, the Anophelesmosquito, is around to introduce the parasite into the humanbloodstream.

The vector does not have to be biological. Many infectious diseases are transmitted by droplets which enter the airway (e.g. common coldand tuberculosis).

Mortality from infectious diseases

The World Health Organizationcollects 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 pneumonias) and diarrheal diseases are caused by many different viruses, bacteriaor parasites.

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.

• The Influenza Pandemic of 1918 or Spanish Flukilled 25 to 40 million in 1 year (about 2% of world population of 1.7 billion).
  • Influenzanow kills about 250,000 to 500,000 worldwide each year.
• The Black Deathof 1347 to 1352 killed 25 million in Europe over 5 years (estimate of 25 to 50% of Europe, Asia, and Africa - world population was 500 million).

New diseases and pandemics

In many 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 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 microbecan take up residence in a new ecological niche.Once that happens, a pathogenthat 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.

Pathogen strains which cause a milder illness are selected and eventually strains well adapted to the human hosts and causing milder illness predominate. It is to the microbe's genes' advantage to do so, because if a disease is rapidly fatal, the host is likely to die before the microbe can get passed along to another host. 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 diagnosisand therapyof infection.

Diagnosis

Diagnosis is initially by medical history and physical examination, and imaging (such as X-rays), but the principal tool in infectious disease is the microbiological culture. In a culture, a growth mediumis 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 Staphylococcusor Streptococcus.

Certain agents cannot be cultured, for example the above-mentioned Treponema pallidum and most viruses. The first serologicalmarkers were developed to diagnose syphilis(the Wassermann test, later replaced by the VDRLand TPHAtests). Serology involves detecting the antibodiesagainst an infectious agent in the patient's blood. In immunocompromisedpatients (e.g. AIDS), serologycan be troublesome, because the antibody reaction is blunted.

A more recent development is direct detection of viralproteins and/or DNA in bloodor secretions. 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 agentis 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 Mycobacteriaas 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 spirocheteof syphilis, cannot be culturedin vitro - however the organism can be cultured in rabbit testes].

Epidemiologyis another important tool used to study disease in a population. For infectious diseases it helps to determine if a disease outbreakis 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 culturehas proven to be positive, the sensitivity (or, conversely, the antibiotic resistance) of an agent can be determined by exposing it to test doses of antibiotic. This way, the microbiologistdetermines how sensitive the target bacteriumis to a certain antibiotic. This is usually reported as being: Sensitive, Intermediate or Resistant. The antibiogram can then be used to determine optimal therapy for the patient. This can reduce the use of broad-spectrum antibioticsand lead to a decrease in antibiotic resistance.

The work of an infectiologist

Doctorswho specialise in the medical treatmentof infectious disease are called infectiologists or infectious disease specialists. Generally, infectionsare initially diagnosed by primary carephysicians or internal medicinespecialists. For example, an "uncomplicated" pneumoniawill generally be treated by the internistor the pulmonologist(lung physician).

The services of the infectious disease team are called for when:

• The disease has not been definitively diagnosed after an initial workup
• The patient is immunocompromised(for example, in AIDSor after chemotherapy);
• The infectious agentis 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.

The work of the infectiologist therefore entails working with patients and doctors on one hand and laboratory scientistsand immunologistson the other hand.

History

Anton van Leeuwenhoek(1632-1723) advanced the science of microscopy, allowing easy visualization of bacteria.

Louis Pasteurproved 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 Salkand Albert Sabindeveloped successful vaccines for Smallpoxand polio, reducing the threat of these debilitating diseases.

See also

• Infection
• Microbiology
• List of infectious diseases
• Copenhagen Consensus
• Important publications in infectious diseae
• Big killer

References

• Mandell's Principles and Practices of Infection Diseasesor this site
• Manson's Tropical Diseases
• Control of Communicable Diseases Manualedited by James B. Chin, APHA, 2000
• Encyclopedia of public healthedited by Lester Breslow, Macmillan Reference 2002
• Table: The World Health Report- 2004 Annex Table 2 and 1995 Table 5

External link

• IDsociety- The Infectious Disease Society of America
• GIDEON- Global Infectious Diseases and Epidemiology Network
• EPIDEMIca- A weblog about infectious diseases.
• When Insects Spread Disease, discusses problem and offers tips for prevention

eu:Gaixotasun infeziosoak es:Enfermedad infecciosa fr:Maladie infectieuse lt:Infekcin? liga nl:Infectieziekte id:Penyakit infeksi ja:??? pl:Choroba zaka?na pt:Doença infecciosa ro:Boal? infec?ioas? simple:Infectious disease zh:???

This article is licensed under the GNU Free Documentation License.
It uses material from the http://en.wikipedia.org/wiki/Infectious+diseases Wikipedia article Infectious diseases.