Final answer:
Scurvy increased in the fourteenth and fifteenth centuries due to economic decline, poor nutrition, lack of access to foods high in vitamin C, and ignorance of dietary needs during long sea voyages. The disease became widespread among sailors until the adoption of citrus fruits in their diet after James Lind's discovery.
Step-by-step explanation:
The increase of scurvy during the fourteenth and fifteenth centuries can be attributed to a variety of factors. Notably, economic decline and resultant poor nutrition weakened people's immune systems. Malnutrition, limited access to fresh foods rich in vitamin C such as citrus fruits, and the lack of understanding of dietary needs in prolonged sea voyages exacerbated the problem. Sailors and people who were lower in the economic spectrum could not afford or did not have access to the fresh fruits that prevent scurvy, leading to an increase in the disease.
In the fifteenth and sixteenth centuries, European exploration and colonisation required long sea voyages. The preservation of fruits was difficult on such voyages, and the sailors' diets were severely deficient in vitamin C, causing outbreaks of scurvy. It was not until the work of James Lind in the eighteenth century that the connection between citrus fruits and scurvy prevention became widely accepted, leading to the British Royal Navy's adoption of citrus fruits in sailors' diets to prevent the disease.