The Irish Potato Famine began in the middle of the nineteenth century and by the end of that century, the population of Ireland had been reduced by half. The rural poor in Ireland (the vast majority of the population at the time) had become dependent on the potato to sustain themselves, and when the crop failed due to potato blight, people starved by the millions or emigrated in desperation to escape the same fate. This incident provides many lessons (not least of all to resist the urge to become dependent on one seemingly super-crop) and continues to inform political relations to this day (e.g. between Ireland and Great Britain, and Ireland and the United States).

When the Europeans arrived in the New World, the population of the Americas was somewhere between thirty and one hundred million people. (Scholars frequently disagree over this statistic.) They arrived bringing a type of systematic violence unfamiliar to many of the native people, and these atrocities were responsible for significant loss of life, but far more native people were killed by the diseases that the Europeans brought with them. When people have had no exposure to a disease in their history, they will not have developed any natural immunities or defences to it; this makes them much more vulnerable to the disease than the carriers. The Europeans had been living with, and dying from, smallpox for centuries, but the Native Americans had not. The disease spread rapidly through North and South America, usually faster than the Europeans could move themselves, and decimated the native population. By some estimates, as many as ninety percent of the native population succumbed to the disease.

The Black Death was carried into Europe in the 1300s by fleas on rats aboard trading ships arriving in the powerful Italian city-states. The continent had experienced previous incarnations of “the plague” and would experience several more in the centuries to come, but this particularly incarnation spread like wildfire through the European population. Within five years the population of Europe was reduced by one-third; whole towns and regions were lost to history.

Both of these words are used to describe infectious diseases. However, they have very different meanings. A disease that is “endemic” is commonly found within a certain area, but not commonly found outside that area. Whereas a “pandemic” is a disease that is widespread, even global in nature, and threatens all people regardless of where they live. The Spanish Flu of 1918 is one of the most famous recent examples of a “pandemic.” You might remember the recent Ebola crisis was “endemic” to several countries in West Africa, but many feared that the disease would become “pandemic” so people returning from those areas were quarantined and meticulously monitored.

The difficulty of crossing the Mediterranean Sea causes many hundreds of African migrants to perish every year in their attempts to make a new life for themselves in the European Union. Anything that acts as limiting factor or a deterrent to migration is defined as an “intervening obstacle” by geographers and demographers.

Chronic diseases, also referred to as degenerative diseases, are "old age" diseases that involve long-term deterioration. Influenza is an example of an infectious disease in that it is passed by direct transmission, while all of the other answer choices are not directly transmitted. 

The difference between an epidemic and a pandemic is that a pandemic refers to a disease that has spread worldwide, while an epidemic refers to a disease that has only spread regionally. An example of a pandemic is the Spanish influenza virus of 1918 that killed over 40 million people.