In medicine, infectious disease or communicable disease is disease caused by a biological agent (e.g. virus, bacterium or parasite), as opposed to physical (e.g. burns) or chemical (e.g. intoxication) causes.

Agents and vectors

Infectious disease requires an agent and a mode of transmission (or vector). A good example is malaria, which is mainly caused by the parasite 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 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 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 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).

The science 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.

Epidemiology is another important tool used to study disease in a population. For infectious diseases it help 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).

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-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.

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.

Jonas Salk and Albert Sabin developed successful vaccines for polio, effectively ending the threat of this debilitating disease.

References

• 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 (http://www.who.int/whr/en/) - 2004 Annex Table 2 and 1995 Table 5

External link

• GIDEON (http://www.GIDEONonline.com) - Global Infectious Diseases and Epidemiology Network