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INFECTION

Disease process that results from the invasion of an organism by bacteria, viruses, fungi, protozoa (e.g.malaria), or metazoa (certain worms).The human body is in contact with many organisms but these do not necessarily harm it; for instance a number of batcteria live in the mouth and alimentary tract and do not normally cause disease, for they live in a symbiotic relationship with their host. They only cause disease if their numbers increse dramaticaly or if the normal defence mechanisms of the host are substantially lowered. Other organisms are much more virulent and if they enter body they nearly always cause diease.

The way of entry of infecting organisms varies considerably but in the epidemiology of disease it is important to know the various routes by which they are transmitted from one individual to another. In many bacterial and virus diseases droplet infection by the mouth and nasal passages is the usual mode of spead. Diseases such as the common cold, influenza, measles, diphteria, and whoopig cough are all transmitte in this way. Othr dieases are caught becauase the organism is transmitted to the mouth by contaminated hands: the contamination is often faecal. Infective hepatitis, dysentery, typhoid, and polio may be tranferred in this way. In certain instances major epidemics of disease such as typhoid may be due to contamination of the water supply by a carrier of germ. One occasions other routes of infection are use: for instance the bite of a mosquito (in a district where malaria is endemic) transmits malaria parasites into the blood stream and certain worms may penetratae the feet of people who walk bare footed in tropical countries. Infections such course, and sometimes disease is caught by the use of dirty syringes.

Once the organism has entered the body it multiplies. How it does this depends on its nature. Viruses live within the cells, destroying them as they increase in number. Other organisms are usually extracellular. Many organisms have a pre dilection for certain sites in the body such as the lungs or alimentary tract, although in many instances they may be found in the blood stream for a short priod of time. This is the phase of bacteraemia or viraemia. Persistent infection of the blood with bacteria is called septicaemia. The incubation priod of an infection varies with the organism.

The harmful effects of an organism may be due to the bacteria or virus itself or to certain toxins it may produce. Thus in diphtheria the inflammatory reaction produced by the diphteria organism causes a membrane to appear which may obstruct respiration. In addition an exotoxin is produced which circulates in the blood stream and damages heart and muscle and may also affect the nervous system.

The body combats infection in various ways. White blood cells (leucocytes) are carried in the blood stream to the site of a bacterial infection. There they attempt to engulf the invading organisms, and if possible to wall them off from the rest of the body. Granulation tissue and fibrous tissue may be formed to provide a physical barrier against the spread of infection. The reticuloendothelial system responds to the presence of invading organisms or theri toxins by producing antibodies, whose chemical structure may enable them to lock on to the foreign molecules and effectively neutralize them.

Whether the body's defences are successful or not depends on many factors, such as the number of infecting organisms present and their virulence, the general health of the patient, and the level of white cells and of gamma globulin available for the formation of antibodies. In patients with agranulocytosis (in which the white cell count is very low) or with agammaglobulinaemia (in which the gamma globulin fraction of the serum proteins is deficient) almost any infection may be over-whelming.

The other factor that affects the response to infection is medical treatment with such drags as antibiotics or sulphonamides. The course of many infections has now been greately modified by the use of these preparations, which will often terminate a bacterial infection even before the body's defences have become fully mobilized. Chemo-therapeutic agents are available for use against all the groups of infecting organisms named above, with the exception of viruses; work on many drugs was still in progress.

Compiled by C.K.Mohamed/Tellicherry