Infectious disease
In medicine, infectious disease is disease caused by a biological agent, 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 caused by the parasite Plasmodium falciparum but does not affect humans unless the vector, the Anopheles mosquito, is around to introduce the Plasmodium into the human bloodstream.
The vector does not have to be biological. Many infectious diseases are transmitted by droplets from the airway (e.g. common cold and tuberculosis).
The Science of Infectious disease
To prove whether a disease is of infectious nature, it has to answer Koch's postulates (Robert Koch), namely 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 has proven to be very hard to meet some of the criteria. For example, Treponema pallidum, the causative spirochete of syphilis, cannot be cultured.
Diagnosis and therapy
The science of infectious diseases also occupies itself with the diagnosis and therapy of infection.
Diagnosis
Diagnosis can be through microbiological culture, i.e. providing a growth medium for a particular agent and determining whether there is growth on the media. This works for a number of bacteria, for example Staphylococcus or Streptococcus.
Certain agents cannot be cultured, for example the abovementioned Treponema pallidum and most viruses. The first serological markers were developped to diagnose syphilis (the Wassermann test, later replaced by the VDRL and TPHA tests). Serology generally means detecting the antibodies against an infectious agent in the (suspected) patient's blood. In immunocompromised patients (e.g. AIDS), serology can be troublesome, because the antibody reaction is blunted.
A more recent is direct detection of viral proteins and/or DNA in blood or secretions. The latter can be done by PCR (polymerase chain reaction), i.e. the amplification of viral DNA and the subsequent detection with anti-DNA probes.
Therapy
When a culture has proven to be positive, the sensitivity (or antibiotic resistance) of an agent can be determined by exposing it to test doses of antibiotic. This way, the microbiologist determines whether a bacterium is [S]ensitive, [I]ntermediate or [R]esistant to a certain antibiotic. 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.
History
Louis Pasteur proved beyond doubt that certain diseases can be caused by infectious agents.
Robert Koch, mentioned above, gave infectious disease a scientific basis by formulating Koch's postulates.