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Chapter: Medical Microbiology: An Introduction to Infectious Diseases: Host-Parasite Relationships

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Emergence of Infectious Disease

The presence of human populations is large enough to sustain and amplify parasites, thus contributing to increased disease.

EMERGENCE OF INFECTIOUS DISEASE

The presence of human populations is large enough to sustain and amplify parasites, thus contributing to increased disease. Humans have lived in communities large enough to per-petuate parasites only for about 10,000 years, barely a blink of the eye in the time frame of evolution. Thus, many of the human diseases that have been predominant historically probably did not exist in early humans. Many of the well-known infectious diseases of humans are very recent in the evolutionary sense. For example, the great Black Death of the 14th century, just 700 years ago, led to the death of approximately one third to one half of the known human population. The effects of plague on the human population are still largely unknown. In terms of the evolution of the human gene pool, those that died were likely as important as those that survived. It has been suggested that the resistance of some Caucasian populations to the recent scourge of human immunodeficiency virus (HIV) may actually reflect the genetic consequences of survival from some infectious dis-ease prevalent 20 generations ago. However, some diseases such as treponematosis, my-cobacterial infection, infections caused by some protozoans and worms, and diseases caused by herpesviruses, likely afflicted early humans because of their latency and their tendency to reactivate over long periods of time.

Poverty, with its crowding, unsanitary conditions, and often malnutrition, leads to an increased susceptibility to infection and disease. War, famine, civil unrest, and, of course, epidemic disease lead to a breakdown in public infrastructure and the increased incidence of infectious diseases.

In the history of human civilization, one of the most important facets of the evolution of human infectious diseases was the domestication of animals, which began about 12,000 years ago. There is good cause to think many of the best-known epidemic diseases evolved from animal species and only became adapted to humans rather recently. We are still in an evolutionary dynamic with our large and small parasites; the relationship be-tween humans and the microbes they are heir to has not stopped evolving. Perhaps it never will. While microbes have evolutionary flexibility, humans try to meet the onslaught of infection with genes that are essentially still those of primitive hunter-gatherers. The ac-tual large-scale domestication of animals has slowed, and it has been replaced by the en-croachment of human populations into the domain of animal, insect, and marine species all over the globe. It is little wonder that our deliberate destruction of predators and the outgrowth of human populations into previously virgin land with its attendant destruction of habitat lead to the emergence of “new” diseases such as Lyme disease; Legionnaires’ disease; and likely, acquired immunodeficiency syndrome (AIDS).


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