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 mainly 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
The science of
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 field of infectious diseases also occupies itself with the
diagnosis and therapy of infection.
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, 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
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 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.
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.
who specialise in the medical treatment of infectious disease
are called infectiologist or infectious disease specialist. Generally,
infections are diagnosed by primary care physicians or internal
medicine specialists. For example, an "uncomplicated" pneumonia will
generally be treated by the internist or the pulmologist (lung
type of food borne illness occur when the food
eaten is contaminated with living pathogenic bacteria.
type of bacteria will determine the time for symptoms to appear. The
bacteria will pass through your stomach and down into your lover
intestine. It will embed themselves in the wall of the intestine and it
infection most often cause diarrhea (sometimes
bloody), stomach cramps and fever. More...
infection is, in effect, a war in which the infecting organism seeks to
utilize the host resources in order to multiply at the expense of the
The difference between an infection and a colonization is often only a
matter of circumstance. Organisms which are normally non-pathogenic can
become pathogenic under the right conditions, and even the most
virulent organism requires certain circumstances to cause a
compromising infection. More...