Infectious disease
In
medicine,
infectious disease or
communicable disease is
disease caused by a
biological agent such as by a
virus,
bacterium or
parasite. This is contrasted to
physical causes, such as
burns or
chemical ones such as through
intoxication.
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
memes 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
parasites, 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).
The
World Health Organization collects information on global deaths by . 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.
| 2002 | 1993 | | World population | 6.2 billion | 5.5 billion |
| Total deaths from all causes | 57 million | 100% | 51 million |
| Rank | Cause of death | Number | Percentage of total | Number | 1993 Rank |
| I. Communicable diseases category | 14.9 million | 26.0% |
| 1:Lower respiratory infections | 3.9 million | 6.8% | 4.1 million | 1 |
| 2 | :HIV/AIDS | 2.8 million | 4.9% | 0.7 million | 7 |
| 3:Diarrheal diseases | 1.8 million | 3.2% | 3.0 million | 2 |
| 4:Tuberculosis (TB) | 1.6 million | 2.7% | 2.7 million | 3 |
| 5:Malaria | 1.3 million | 2.2% | 2.0 million | 4 |
| 6:Measles | 0.6 million | 1.1% | 1.1 million | 5 |
| 7:Pertussis | 0.30 million | 0.5% | 0.36 million | 7 |
| 8:Tetanus | 0.21 million | 0.4% | 0.15 million | 12 |
| 9 | :Meningitis | 0.17 million | 0.3% | 0.25 million | 8 |
| 10:Syphilis | 0.16 million | 0.3% | 0.19 million | 11 |
| 11:Hepatitis B | 0.10 million | 0.2% | 0.93 million | 6 |
:Tropical diseases (6)| 0.13 million | 0.2% | 0.53 million | 9, 10, 16, 17, 18 |
(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. 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.
Lower respiratory infections, which include various
pneumonias, and diarrheal diseases are caused by many different
viruses,
bacteria or
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
Chagas disease,
dengue fever,
lymphatic filariasis,
leishmaniasis,
onchocerciasis,
schistosomiasis and
trypanosomiasis.
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 Flu killed 25 to 40 million in 1 year (about 2% of world population of 1.7 billion).
**
Influenza now kills about 250,000 to 500,000 worldwide each year.
* The
Black Death of 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 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 andâ€"simply because it is new to the human speciesâ€"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.
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-rays), 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
viruses. 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
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 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
culture has 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
microbiologist determines how sensitive the target
bacterium is 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 antibiotics and lead to a decrease in
antibiotic resistance.
The work of an infectiologist
Doctors who specialise in the
medical treatment of infectious disease are called
infectiologists or
infectious disease specialists. 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 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
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.
The work of the infectiologist therefore entails working with patients and doctors on one hand and
laboratory scientists and
immunologists 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.
Alexander Fleming discovers the world's first antibiotic Penicillin which in turn inspired the discovery of the other
antibiotics available today.
Gerhard Domagk develops
Sulphonamides, the first broad spectrum synthetic antibacterial drugs.
*
Infection*
Microbiology*
List of infectious diseases*
Copenhagen Consensus*
Important publications in infectious diseae*
Big killer*
Mandell's Principles and Practices of Infection Diseases or
this site*
Manson's Tropical Diseases*
Control of Communicable Diseases Manual edited by James B. Chin, APHA, 2000
*
Encyclopedia of public health edited by Lester Breslow, Macmillan Reference 2002
* Table:
The World Health Report - 2004 Annex Table 2 and 1995 Table 5
*
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
