Infectious diseases are the most common ones on Earth. In different countries, the prevalence and features of infections are different, which is largely influenced by socioeconomic factors. In low-income countries, there are more infectious diseases than in industrialized countries with better social conditions, prevention of diseases and well-organized public health education. However, outbreaks of infectious diseases and epidemics emerge even in countries with very high civilization levels. Pandemics periodically spread worldwide, affecting both low-income and rich countries.
There is one more feature of infectious diseases: they do not disappear forever. Besides, new infectious diseases constantly emerge as well. Thus, at the end of the 20th century, HIV infection and AIDS-related mortality emerged and began to spread rapidly. At the beginning of the 21st century, the population of Earth was affected by the COVID-19 coronavirus pandemic. However, natural and induced pathomorphosis of infectious diseases occur under the impact of medical, social, ecological, and other effects, while their course and outcomes become more favorable.
Natural pathomorphosis presumes changes in the general panorama of infectious diseases. Thus, at the end of the 20th century, several new diseases emerged, especially viral ones (HIV, hemorrhagic fevers, COVID-19 coronavirus), infections caused by microorganisms including Legionellae, etc. The spectrum of infectious diseases has changed as well: viruses have become predominant. Morbidity due to tropical infections and infectious diseases caused by opportunistic microorganisms has increased significantly.
Induced pathomorphosis of infectious diseases is related to complex long-term sociomedical reforms. Thus, morbidity related to pediatric infections has drastically decreased at the end of the 20th century. Currently, there is no smallpox worldwide. However, the history of mankind controlling infectious diseases demonstrates that their induced pathomorphosis is not stable. Seemingly, controlled infections can return when socioeconomic conditions worsen.
To understand the essence of infectious diseases, one should view them from general biological positions, taking into account the fact that microorganisms are representatives of the human environment. The human body has natural protection against the effects of pathogenic microflora in the form of anatomic barriers: the skin and mucous membranes covered by secret with antimicrobial properties (lysozyme, several organic acids). Physiological barriers are also present including mucociliary clearance, cough, acidic reaction of the gastric juice, bowel peristalsis, constant urine flow, etc. Finally, immune barriers are very important: resident cells of the immune system (lymphocytes, macrophages) in the derma, on the epithelial surface of mucous membranes, in pulmonary alveoli, and in blood vessels. Lymphoid tissue is located in the lamina propria of mucous membranes, while IgA secretion occurs at the surface epithelium.
An infectious disease essentially is a conflict of the organism with the environment, or a special form of symbiont relations of macro- and microorganisms (Gamaleya N.F.). However, normal life of humans and animals requires symbiont relations between macro- and microorganisms. The human body is populated by masses of microbes, which have specific interactions. For example, a normal intestinal function is impossible without Escherichia coli. If the doctor forgets about it and administers drugs that harm the intestinal flora, the patient develops dysbiosis leading to severe consequences. Bacteria are constantly present in the oral cavity, trachea, bronchi, genitals, etc. They create an optimal pH of the medium that helps tissues and organs to perform their physiological functions. Specific microorganisms may participate in metabolism themselves. Thus, symbiosis, relations of nonsusceptibility (acquired and congenital immunity) exist between the human and many microorganisms. This nonsusceptibility condition mainly defines the state of health.
However, symbiosis does not always characterize the organism nonsusceptibility to causative agents. This is a rather comprehensive term including mutualism (mutual benefits of the causative agent and the host), commensalism (benefit of a single partner), and parasitism (antagonistic symbiosis doing harm to the host).
An infectious disease is caused by microorganisms which lack any symbiont relations, or when these are somehow impaired. Special terms exist that describe features of interactions between macro- and microorganisms.
An infectious disease or infection (Latin infectum) is microorganism invasion and reproduction in the macroorganism with the development of different interaction forms from carrier states to manifest disease.
Infectious process is a complex of macroorganism responses to the invasion and reproduction of pathogenic microorganisms, aiming to restore homeostasis and equilibrium with the environment.
Susceptibility to infection is the ability of a human or animal to respond to causative agent invasion with the development of a disease or carrier state.
The following groups of infectious diseases are described:
- anthroponoses, diseases typical of humans only;
- anthropozoonoses, diseases affecting both humans and animals;
- biocenoses, infectious diseases of humans developing through an intermediate host. Malaria is a classic example of biocenosis. Its infection develops as follows: diseased human - Anopheles mosquito - healthy human. Biocenoses develop in locations where the intermediate host resides, a representative of animal world existing in a specific region. Thus, biocenoses are identified as a geographic pathology, and they belong to infectious diseases with natural foci (Pavlovskyi E.N.).
An infectious disease is impossible without the causative agent, as it is the cause of the disease. Depending on the etiology, infectious diseases are divided into prion, viral, mycoplasmic, rickettsial, bacterial, fungal, protozoal, parasitic, and arthropod-related.