Diseases of man due to pathogenic microorganisms are studied by a special branch of medicine. In the middle of the 19th century these diseases were given the name of infectious, from Latin inficere, which means to stain or to taint with morbid matter, i.e., to contaminate.
As distinct from other human pathologies, infectious diseases are characterized by specificity of the living microorganism causing the disease, its transmissibility from a diseased human or animal to healthy people, epidemic dissipation among population under certain conditions, the cyclic character of the clinical course with specific symptoms characteristic of a given particular disease, development of immunity in those who sustained the disease, development of allergy to a given causative agent, and persistence of the carrier state in some infectious diseases after clinical recovery.
As a rule each particular disease is caused by its specific causative agent. Besides, many pathogenic microorganisms can be retained in certain organs or tissues (tropism), where they find beneficial conditions for their multiplication. Accordingly, the clinical picture of some infectious diseases is characterized by specific symptoms of involvement of separate organs and systems. For example, dysentery (shigellosis) is characterized by the presence of mucus and blood in the stools which is due to inflammation of the mucosa of the large intestine; catarrh of the airways is characteristic of measles and influenza.
The site of entry of the pathogenic microorganism to the body is called the portal of infection. In some infectious diseases, such dysentery, or cholera, the causative microorganisms enter the body
only through one certain gateway, while in other infection they can enter the microorganism through various portals, e.g. in brucellosis, tularaemia, or plague.
As the microorganism enters the human body it can multiply at the site of its entrance (i.e., in the portal of infection) to evoke a pathology. The microorganisms produce toxins which act on the infected body. Besides, the causative agents can be disseminated inside the macroorganism by various routes, e.g. with lymph, blood, or by nervous fibres. From the foci of their multiplication, the agents enter the blood to cause bacteraemia (circulation of bacteria in the blood), virusaemia (the presence of virus in the blood), or toxaemia (the presence of microbial toxins in the blood).
Toxins are classed as endotoxins and exotoxins. Agents of botulism, tetanus or diphtheria produce exotoxins which act selectively on various tissues and organs, while endotoxins are liberated during destruction of microbial cells; the latter toxins are less specific.
Infectious diseases are characterized by the staged character of their course and by a certain sequence of its stages, in which the symptoms increase or decrease. The following periods are distinguished: incubation (latent) period, prodromal period (the period of precursors), the main clinical manifestations of the disease, and the recovery phase (convalescence).
The incubation period lasts from the time of ingress of the infection into the macroorganism till the time when the first clinical symptoms become manifest. Each disease has its specific incubation period during which the clinical symptoms are absent. The pathogenic agent multiplies and disseminates in the body during the incubation period, the length of which varies from few hours (food poisoning) to several days (plague, cholera, typhoid fever), weeks (viral hepatitis A), months (viral hepatitis B, rabies), and even years (leprosy).
The incubation period is superseded by the prodromal period, which lasts from several hours to several days. The first symptoms of the disease develop during this period. These symptoms are, e.g. headache, malaise, slightly elevated body temperature, myalgia, loss of appetite, catarrh, gastrointestinal dysfunction, and the like. A correct final diagnosis cannot be established at this stage.
The period of main clinical manifestations lasts from several days (measles, influenza) to several weeks (typhoid fever, brucellosis). This period is divided into the following phases: increasing clinical symptoms, the period of advanced disease in its full swing (develop-, ment of the symptoms specific for a given disease), and decreasing clinical manifestations of the pathology.
The clinical symptoms can decrease gradually, during 4-5 days (lysis), or terminate suddenly with an abrupt fall of body temperature within few hours or 1-2 days (crisis). Exacerbations can develop during the period of decreasing clinical symptoms.
The period of the main clinical manifestations is followed by the recovery phase (convalescence). The length of this period depends on the immune reaction of a given patient, the severity of the disease, efficacy of treatment, and on some other causes. Recovery can be complete or incomplete. Residual phenomena can be seen during incomplete recovery.
Relapses of the disease can occur in some infectious diseases after recovery: the entire clinical picture can be repeated in 5-20 days, but the course of the relapse will be shorter than of the main disease. Some diseases, e.g. tuberculosis, brucellosis, or dysentery can run a protracted and sometimes chronic course, and persist for years.
Infectious diseases can be followed by specific complications. Otitis, lymphadenitis or nephritis can complicate scarlet fever, while typhoid fever can be complicated by intestinal haemorrhage or perforation of the intestine; some complications can be non-specific, i.e., caused by some other microorganisms. The most dangerous complications of infectious diseases are shock, renal encephalopathy (viral hepatitis), acute renal failure (meningococcal infection), brain oedema (meningitis), etc., which require intensive therapy.
Infectious diseases caused by one agent are called monoinfections, while infections due to several agents-mixed infections.
Mixed infection should be differentiated from secondary infection. In the latter case, a new infection is superimposed upon the existing infection (e.g. an infection due to staphylococcus).
Reinfection is a repeated disease state due to new invasion of the same causative agent (scarlet fever). If a patient is reinfected before his convalescence by the same causative agent but belonging to another type, the case is referred to as superinfection (dysentery, malaria).
Symptoms of infectious diseases. The clinical manifestations of infectious diseases are varied, but some clinical symptoms are common for all of them. The main symptom that can be seen in almost all infections is fever, that develops due to the upset thermal regulation caused by toxic substances of bacterial and tissue origin. Body temperature can be subfebrile, from 37 to 37.9 °C. moderate, from 38 to 39 °C, high (pyrexia), from 39 to 39.5 °C, hyperpyretic, above 40 °C, and subnormal, below 36 °C.
Fever can also be different by its character (Fig. 7): it can be'
Fig. 7. Temperature curves:
a-continuous fever; b-remittent fever; c-intermittent fever
Fig. 7. Temperature curves:
a-continuous fever; b-remittent fever; c-intermittent fever continuous, with variations not exceeding 1 °C in twenty-four hours; remittent, with diurnal variations over 1 °C (sometimes 2-2.5 °C); intermittent, with alternation of paroxysms of fever with periods of normal body temperature; undulant, with wave-like rises and falls of the temperature curve within several days or even weeks; recurrent or relapsing fever, with alternation of 4-7 day long periods of fever arid apyrexia.
Many infectious diseases are attended by changes in the skin and visible mucosa. The skin becomes dry or, on the contrary, covered with sweat. Various rash can also develop (exanthem). The morphology of skin eruptions is different and classed as follows.
Roseola is a rose-coloured rash, with elements sizing from a millet grain to the size of a pea. Roseola disappears when pressure is exerted but reappears in 2-3 seconds after release of pressure. This is dilatation of a minutest blood vessel.
Petechia is a pinpoint brown-red spot: it does not disappear when pressed, because it is due to intradermal haemorrhage.
Bulla is formed due to exudation into the surface layer of the skin. It occurs in serum disease and urticaria.
Erythema is a large rose-coloured or red spot, often raised over the skin surface.
Papule is a red nodule raised over the skin surface due to infiltration of cell elements.
Vesicle is a small bladder filled with clear serous fluid.
Pustule is a vesicle filled with purulent fluid circumscribed by a rim.
After resolution of some forms of rash, the underlying skin
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