Did scurvy exacerbate the plague?

Plague, the dreaded infectious disease, that decimated the populations of cities, countries and empires. It is caused by the bacterium Yersinia pestis. The plague is generally spread by rats that carry infected fleas.

Several pandemics are known to have ravaged the historic world. Three main Yersinia pestis strains are recognised: (1) Yersinia pestis Antiqua, which caused the Justinian Plague (541–544 AD), but continued intermittently until ∼750 AD; (2) Yersinia pestis Medievalis, which caused the Black Death in Europe (1347–1351 AD) and included successive waves, such as the Great Plague (1665–1666 AD), until the 18th century; and (3) Yersinia pestis Orientalis, which emerged in China in the 1850s and erupted there in a major epidemic in 1894 before spreading across the world as a series of epidemics until the middle of the 20th century.

Plague outbreaks appear to be related to increased abundance of rodents and other mammals that serve as hosts for the fleas that then transmit the disease to humans. We know that the plague is partially seasonal: at the end of the harvesting season, rat populations drop because they don't have enough to eat[1][2]. As a result, fleas start looking for new hosts to bite, and so begin targeting humans.

Most plague pandemics are preceeded by dramatic weather events, causing misery and crop failures. The result was famine.

But in times of famine, there is hardly any food for both humans and rats. As prolonged bad weather was seen over entire continents, rat populations dwindle accordingly over these continents, explaining the extremely fast expansion of the plague.

The population was already weakened by famine, but the prolonged lack of fresh fruits and vegetables meant that Vitamin C deficiency would be astonishingly common and that would undoubtedly lead to rampant scurvy. Remember, it was not uncommon for over 80% of sailors to die from scurvy during long voyages, with a total of an estimated two million succumbing to it between 1500 and 1800.

It's a deadly combination: famine, scurvy and the plague. No wonder then that the Black Death is estimated to have killed 30–60% of Europe's total population. The plague was somewhat less common in parts of Europe that probably had some alternative vitamin C-rich food supplies, such as seeweeds[3] (...vitamins such as vitamin C are present in large amounts in Ulva lactuca (sea lettuce).... That means that the majority of the Basque Country, isolated parts of Belgium and the Netherlands would have a highter chance to survive.

[1] Eckert: Seasonality of plague in early modern Europe: Swiss epidemic of 1628—1630 in Reviews of Infectious Diseases – 1980
[2] Bacaër: The model of Kermack and McKendrick for the plague epidemic in Bombay and the type reproduction number with seasonality in Journal of Mathematical Biology – 2012
[3] MacArtain et al: Nutritional value of edible seaweeds in Nutrition Reviews - 2007