The probability of an outbreak of an infectious disease depends on the state of immunity of the population, the pathogenic properties of the pathogen, and the epidemic characteristics of the disease. For example, outbreaks of meningococcal disease occur on average every 10-12 years. In the “meningitis belt” in Africa, the incidence can reach 1 thousand people for every 100 thousand people.
Endemic infections are called endemic, characteristic of a certain territory. However, the number of infected people can vary. Sometimes epidemics occur – a massive and progressive spread of infectious diseases within a certain territory, significantly exceeding the usually recorded incidence rate in this territory for a similar period of time.

The emergence of new pathogens of infectious diseases
The causative agents of some serious infectious diseases, registered recently, are completely new microorganisms. The most important example is the human immunodeficiency virus (HIV). It has much in common with the simian immunodeficiency virus, and therefore there is an assumption that about fifty years ago in Central Africa there was a transmission of this virus from chimpanzees to humans, and its adaptation to the conditions of the body of a new host.
Over time, the number of infected people gradually increased, and the virus was able to spread throughout the world. Another example is a previously unknown coronavirus, also obtained from animals in southern China and causing the severe acute respiratory syndrome.
Medical intervention or changes in environmental conditions can create favorable conditions for cross-species transmission of pathogens. For example, there is an assumption that a decrease in immunity during xenotransplantation contributes to the infection of humans with animal viruses and their transmission to other people, and the deforestation of the rainforests of South America has led to human infection with Trypanosoma cruzi.

The population of new territories is a cause of new infectious diseases
Climate and demographic changes allow microorganisms to colonize new territories. For example, the West Nile virus is currently actively spreading throughout the United States. Climate change, in particular global warming, contributes to the spread of microorganisms that need a high-temperature environment.

The occurrence of infections that were previously rare
Tuberculosis has become a rare disease in most industrialized countries. At the same time, due to a combination of reasons (migration from countries with high incidence, the spread of HIV infections, neglect of health warnings), the number of cases of tuberculosis began to increase rapidly. After the collapse of the Soviet Union, outbreaks of previously rare diphtheria were reported, due to the lack of vaccination during that period.
All these facts confirm that most of the ” defeated ” diseases at some point can flare up with renewed vigor. In addition, the formation of multi-resistant strains of bacteria contributes to the mass spread of infectious diseases.
Changes in population vaccination programs may allow previously controlled infections to re-emerge and affect different age groups (eg whooping cough and measles in the UK).

Changing agriculture and food industry as a cause of new infectious diseases
Infection by microorganisms of livestock and poultry (for example, strains of Salmonella enteritidis) leads to food contamination and the spread of infection. In developed industrialized countries, the widespread use of food prepared from semi-finished products has led to an increased risk of infection with listeriosis, in connection with which the conditions for storing food products have been revised.

Changing pathogens of infectious diseases
Some pathogens, populating new habitats, cause outbreaks of diseases, displacing other, sometimes related microorganisms. This fact can be explained by the weaker virulence or contagiousness of the latter. Thus, Clostridium difficile (type 027), which produces the strongest toxin, caused an outbreak in Canada, the USA, and the UK, and Acinetobacter baumanni caused many nosocomial infections in London. The spread of methicillin-resistant Staphylococcus aureus types 15 and 16 in most countries has led to the predominance of these strains over all others.

Bioterrorism as a cause of new infectious diseases
Recently, political instability in the world has been increasingly combined with a growing terrorist threat. This has led to an increase in the likelihood of using pathogens of various infectious diseases as biological weapons. The act of bioterrorism using anthrax spores in the United States claimed the lives of four people and demonstrated the need for effective anti-terrorist mechanisms.
It is possible to use the causative agents of smallpox, tularemia, plague, hemorrhagic fever, etc. as biological weapons. That is why health professionals should have timely and complete information about unusual infectious diseases and their symptoms, as well as work in cooperation with all specialized agencies.

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