The Treaty of Tordesillas was an agreement between Spain and Portugal that was signed in 1494, in the namesake town in Spain. In this treaty, Spain and Portugal agreed to divide the New World between themselves for colonization, conversion, and administration by setting a line of demarcation that had run from north to south at a meridian about 370 leagues (one league is equal to three miles) west of the Cape Verde islands. Spain would be granted everything to the west of this line, while Portugal would receive everything to the east. This corresponded roughly to most of eastern modern Brazil and Africa being conceded to Portugal, while Spain would take the rest of the New World. It would later be revised by the Treaty of Zaragoza (1529), which included Asia, since neither party had considered the world's spherical shape at the time of drawing the line of demarcation, and they quickly realized they would need a second line to divide the world into two hemispheres. The Portuguese hemisphere included Japan, as well as the rest of Asia to the west, leaving Spain with most of the Pacific Ocean but none of mainland Asia, with the exception of Spain given the right to trade in the Philippines. Though the treaty was largely ignored by other European powers, particularly when the Protestant Reformation took root, Spain and Portugal followed it fairly faithfully, though it did require some amendments as more of the world was charted.
A letter of marque is a government license that allows the bearer to engage in privateering. Privateers are privately owned ships that are authorized to capture enemy merchant ships
Dysentery, also known as the bloody flux and camp fever, was a very common and significant concern to both Europe and North America. Dysentery is a type of gastroenteritis that causes bloody diarrhea, fever, abdominal pain, and a feeling of incomplete defecation. It's caused by the gram-negative bacterium Shigella dysenteriae, which spreads quickly in unsanitary conditions, usually by contaminated food or water. With respect to Blackthorne's concern about bathing, it is only an issue if the water is untreated.
Portuguese was among the world's key languages in the 15th and 16th centuries, serving as the lingua franca (any language used to establish communication between people whose native languages are different) in several regions, including in African and Asian countries. Portugal was ahead of all European countries when it came to maritime imperialism in the early 16th century, establishing commercial routes and colonies in North, West, and South Africa, and the region that would become known as Brazil. The Portuguese arrived in Japan in 1543, and by the end of the century, both the Portuguese language and the Christian religion had been assimilated by many Japanese. Thus, whenever Blackthrone is speaking in English to a member of the indigenous population, they are supposed to be speaking in Portuguese within the story. Although John Blackthorne is indeed English, he only communicates with the Jesuits through Portuguese, which is also the language used by the translators every time the navigator talks to a Japanese person.
Leprosy, now known as Hansen's disease, is an age-old disease and is described in the literature of ancient civilizations. It is an infectious disease that causes severe, disfiguring skin sores and nerve damage in the arms, legs, and around your body as well as the lungs and eyes. It is spread by airborne respiratory droplets (coughs or sneezes) which contain the bacteria. Leprosy is curable and treatable in the early stages through the use of multi-drug prophylactics. Left untreated, the disease may cause progressive and permanent disabilities. In some cases, leprosy was so virulent, the stricken were placed in leper colonies like Molokai in Hawaii or Carville, Louisiana on the United States Mainland.