Infectious disease overview

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Infectious disease Microchapters


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Historical Perspective




Differentiating Infectious Disease from other Diseases

Epidemiology and Demographics

Risk Factors

Natural History, Complications and Prognosis



Indication of Tests

History and Symptoms

Physical Examination

Laboratory Findings


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Echocardiography or Ultrasound

Other Imaging Findings

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Medical Therapy


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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [2]; Associate Editor-In-Chief: Cafer Zorkun, M.D., Ph.D. [3]


This false-colored electron micrograph shows a malaria sporozoite migrating through the midgut epithelia

An infectious disease is a clinically evident disease resulting from the presence of pathogenic microbial agents, including viruses, 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. [1] 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, foods, body fluids, contaminated objects, aerocole particles, or by vectors.

The term infectivity describes the ability of an organism to enter, survive and multiply in the host; while the infectiousness of a disease indicates the comparative ease with which the disease is transmitted to other hosts.[2] An infection, however, is not synonymous with an infectious disease, as an infection may not cause important clinical symptoms or impair host function.

Epidemiology and Demographics

Epidemiology is an 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 occurrence in a region), epidemic (unusually high number of cases in a region), or pandemic (global epidemic).


History and Symptoms

Diagnosis of infectious disease sometimes involves identifying an infectious agent either directly or indirectly. In practice most minor infectious diseases such aswarts,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.

Laboratory Findings

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. The benefits of identification, however, are often greatly outweighed by the cost, as often there is no specific treatment, the cause is obvious, or the outcome of an infection is benign.

Other Imaging Findings

Techniques (such as X-rays, CT 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.


  1. Dorland's Illustrated Medical Dictionary 2004 WB Saunders.
  2. Glossary of Notifiable Conditions Washington State Department of Health