Has infectious diseases been a factor in human populations?

Emigration from rain forests
humans, Homo sapiens, evolved in Africa 200,000 to 400,000 years ago
something made them leave this seemingly hospitable environment ... perhaps they were trying to "escape" infectious diseases, such as malaria?
diseases such as sleeping sickness may have limited their spread out of the forests into the grasslands in Africa
Elimination of prey and "weeds"
intensive cultivation of agricultural crops became necessary as human populations increased and large-bodied prey were essentially eliminated
they learned to control "weeds" by "slash and burn" in forests, by flooding, and by plowing (developed in the Mediterranean region about 3000 BC)
along with this cultivation of plants for food, humans domesticated a number of animals ... and got diseases from them as the animal's microbes evolved to infect humans
Development of endemic levels of infectious diseases
major civilizations probably developed their own "mixes" of communicable diseases 10,000 to 2500 years ago ... when cities large enough to support endemic diseases arose
large-scale ritual gatherings probably also contributed to the spread of infectious diseases among settlements
due to urban crowding and farming practices in nearby agricultural regions, infections transmitted by water ingestion, skin contact and insect bites probably flourished during these times
as these populations adapted to infectious agents (and the infectious agents to them) over thousands of years, certain diseases became endemic in the cities
once these diseases stabilized, these populations became biological threats to their more distant neighbors because they carried diseases to which they were resistant but to which their neighbors were susceptible
this may have simplified territorial expansion, due to inntroduction of new diseases with expansion into new territories
however, when these civilizations tried to expand into certain regions which had their own reservoirs of infectious diseases, they would have encountered new diseases that limited their expansion
perhaps people have never inhabited some regions (i.e., African Savannas) to any significant degree because the other animals that live there carry infectious agents (such as trypanosomes) that humans have been unable to "adjust" to
early thoughts about cause and transmission of infectious diseases
ancient Egyptians and Indians thought pestilences were punishments inflicted on humans by supernatural beings as retribution for "wicked" behavior
the Hippocratic school (400 BC to 500 AD) believed that diseases resulted from imbalance of the four humors: blood (sanguine), yellow bile (choleric), black bile (melancholic) and mucous (phlegmatic)
Impact of selected diseases
Malaria is transmitted by mosquitoes not by bad ("mal") air
Alexander the Great's armies may have spread malaria as they moved about on their campaigns (Alexander's died of it in 323 BC)
malaria and plague (plus lead poisoning and other factors) contributed to the decline and fall of Rome, because more than 60% of Romans were chronically infected during Julius Caesar's reign in the first century BC
"Roman airs" affected Rome for 500 years, with periodic epidemics, incapacitating most of the farmers , increasing infant mortality and greatly decreasing adult life expectancy
to avoid malaria's effects, they began conscripting the Roman army from conquered Germanic lands (cold) during the fourth century AD
African sleeping sickness is transmitted by tsetse fly bites, and was most likely a major limiting factor in the spread of humans onto the African Savannas (the herbivore herds that live there act as reservoirs)
Schistosomiasis is transmitted by a fluke (flatworm) that lives (for part of its life cycle) in red snails that live in shallow water (rice paddies, etc.) and has clearly been present in Egypt since ancient times
Leprosy has been with us for thousands of years, as indicated by ancient bans on lepers in both Jewish and Old Testament Christian writings
in Europe special houses (called lazars ... after St. Lazarus, the patron saint of lepers) were set aside for lepers as early as 4th century AD, as the Roman government became Christian and adopted biblical injunctions about treatment of persons with disfiguring skin diseases - this is probably where our concept of quarantine came from
leprosy became well established in Europe (France and Britain) and the Mediterranean coastlands (Egypt) during the 6th century AD, then prevalence of this disease declined greatly during the 14th century perhaps because plague and tuberculosis were on the upswing and they may have simply out-competed it because they are "quicker" pathogens
Plague has caused periodic epidemics for thousands of years
a 50-year plague epidemic began in Egypt in 540 AD, then spread rapidly to Alexandria and Palestine via the fleas of rats traveling with traders
it encompassed Europe and Asia, but is referred to as the Justinian plague because it occurred during the reign of this Byzantine emperor
with a death toll of 100 million people, this epidemic stopped not because the bacterium vanished, but because it killed so many that the critical mass of susceptibles needed to sustain it was lost
Plague in the Ancient World
this plague was probably the deciding factor in ushering in the Dark Ages which began with the fall of the Roman Empire
besides playing a major role in the decline and fall of the Roman Empire, this epidemic delayed development of medicine and the germ theory of disease because the Christian church became de facto doctor to both the soul and the body, replacing the progressive Greek and Roman doctors' idea that disease is caused by pathogenic agents with the idea that disease results from vice and sin

Yes, definitely. The black plague in the middle ages killed about 1/3 to 1/2 the people in Europe and Smallpox and Spanish influenza killed many countless people.

Even moreso than you might think. Check out this link and read it for an example of what I mean: http://www.boydgraves.com/timeline/