The Spread of Infection Diseases: Past and Present

(taken from news.bbc.co.uk)

Spanning hundreds of years worth of history, the evolving relationship between humans and the environment has been critical in influencing the emergence and spread of infectious diseases.  The appearance of the first infectious diseases coincided with the adoption of agriculture and the transition to larger settled societies, and only spread with the prospects of trade and travel.  Epidemics are not a thing of the past, but are a relevant, pressing concern in modern societies.  There is a constant dual-evolution between humans and disease: as humans develop new lifestyles and means to control their environments, microorganisms likewise morph to become better adapted to their hosts.

The first great transition in human history occurred when the rise of agriculture and animal domestication lead to the replacement of hunter-gatherer group by sedentary communities.  This major technological advancement and social change also happened to lead to the emergence of widespread infectious diseases.  In the mobile hunter-gatherer lifestyle, human waste was left behind at the campsite while the band moved on and was spared from exposure to potential disease-causing microorganisms.  However, the shift to agriculture and settled communities forced humans to remain in close proximity to their waste products.  The domestication of animals ushered in exposure to a range of animal diseases that had the ability to adapt to humans as new hosts.  The human strains of smallpox, tuberculosis, diphtheria, and influenza all have related cattle and hog strains (Ponting, 226).  Widespread infectious diseases evolved as settled communities gave rise to more populous societies.  The rise of such “crowd diseases” relied on numerous and dense human populations which were previously absent in hunter-gatherer bands and small farming settlements (Jared, 203).  Crowd diseases never arose in small population sizes, since the microbe would have been wiped out once all the hosts were deceased.  Instead, microbes could shift from one local area to another without running out of hosts in the large populations of societies.  This new environment was highly advantageous to microbes, as the great number of people within close proximity and contact with each other provided increased likelihood of infection.

Trade and exploration lead to both the global growth of societies and spread of diseases.  When contact was established between two societies, diseases had a devastating effect on those who had acquired no natural immunity or resistance to it.  As trade increased between Rome and India beginning in 165 AD, a quarter of the Roman population was lost to smallpox (Ponting, 228).  While trade, communications, and travel burgeoned in the 13th century, Eurasia was devastated by the Black Death.  European conquest reached its height in the 14th century and the New World was opened up to the transmission of Old World diseases, and natives such as the Aztecs were destroyed by the smallpox, measles, and typhus brought by the Spaniards.  Once again, the Americas suffered from new foreign diseases during the 16th century when trade with Africa introduced malaria and yellow fever (Ponting, 230).  The human pattern of increasing contact between societies served as a mechanism for microbe propagation. 

Though humans have since devised medical technologies such as medications and inoculations in hopes of combating disease-causing microbes, infectious diseases and epidemics are still present in our modern world.  Societies today still carry on the expansionistic pattern of our past, as new resources are required to sustain our ever-growing populations.  Similar to our past exploration and colonization, our current ongoing urbanization and encroachment into previously untouched habitats (such as the rainforest) make us susceptible to new diseases.  Like the Native Americans in the 14th century, we may be exposed to microbes that we have no endemic resistance towards.  Furthermore, just as oceanic travel radiated the bubonic plague across the Eurasian continent, contact via transportation between societies is responsible for the spread of epidemics today.  The Severe Acute Respiratory (SARS) conavirus epidemic was responsible for 3,096 illnesses and 774 deaths from November 2002 to July 2003 (WHO report).  The respiratory virus originated in mainland China, but through the pathways of trade and accessible air transportation, spread to Canada, the Phillipines, Singapore, Taiwan, and Hong Kong and was declared a global health threat (CDC).

The development of agricultural technologies first introduced the spread of animal diseases to humans and this trend continues to be of prominent concern to this day.  Because of the massive amounts of food required to sustain our population, agricultural techniques now involve close quartering of livestock and poultry.  This extremely dense concentration of hosts is highly advantageous to the microbes, allowing for a rapid rate of infection.  As a result, strains causing mad cow disease can easily spread through the cattle population and make the leap to human infection.  Avian influenza is on the rise, spreading between birds in crowded conditions, such as those used to raise poultry.  Research has shown that the H5N1 virus has become more pathogenic, allowing an infected bird to live longer and transmit the disease to other birds.  The virus has also gained the ability to infect human hosts (CDC).  Modern agricultural technology also relies on the use of antibiotics to treat livestock. Some antibiotics used to treat diseases in cattle, such as cefquinome, are also a “last resort” antibiotic for humans.  The concern is that these microbial “superbugs” may develop a resistance against the antibiotics, which would pose a grave threat human health (Washington Post). 

As humans further adopt and participate in new patterns of expansion, microbes have evolved to match our development.  The adoption of agriculture and the shift from small hunter-gatherer bands to large, densely populated societies created an opportunity for microbes to better adapt to their environment.  Synchronized with our transition, microbes took advantage of the ability to infect more hosts in order to spread more efficiently.  The relationship between human social and technological growth and microbial evolution cannot be halted; as our means of travel, trade, urbanization, and agricultural advancements required to support our growth continues, microbes evolve to match these changes.   

Sources:

Diamond, Jared. 1997. "Chapter 11: Lethal gift of livestock." Guns, Germs, and Steel. W.W. Norton & Co: pp. 195-214

Ponting, Clive. 1991. Chapter 11. “The Changing Face of Death.” Green History of the World. St. Martins Press, NYC: pp. 224-239

http://en.wikipedia.org/wiki/Severe_Acute_Respiratory_Syndrome

http://en.wikipedia.org/wiki/Transmission_and_infection_of_H5N1

http://www.cdc.gov/ncidod/sars/index.htm

http://www.cdc.gov/flu/avian/outbreaks/current.htm

http://www.washingtonpost.com/wp-dyn/content/article/2007/03/03/AR2007030301311_pf.html