254 robert sallares
but also far inland. Although there is very little documentary evidence for the medical
situation in Spain in antiquity, there is abundant evidence from the early modern
period. Malaria dominated medical literature in Spain after the disappearance of
plague until the arrival of yellow fever (Palmero and Vega, 1988; Palmero, 1994;
Núñez, 2003; Rodríguez et al., 2003; Marí and Peydró, 2010). The area around
Madrid, which is very hot in summer, had a reputation for malaria. Don Carlos, the
son of Philip II of Spain, developed the symptoms of quartan fever caused by
Plasmodium malariae at the age of 14 on August 6, 1559. At first he had an irregular
fever which soon adopted the normal periodicity. After 14 months the attacks moved
five hours forward, an unusual feature. The disease lasted until October 1561
(Hernández, 2001).
The most dangerous traveler in the medieval period was, of course, plague. Plague,
a vector-borne disease transmitted by fleas from rodents to humans, had to travel all
the way from east Asia, its probable area of origin according to the conclusions of
research in molecular evolution, to reach Europe, a process doubtless facilitated by
the development of commercial relations between Europe, India and China along the
Silk Road and then the political unification of a large part of Eurasia in the time of the
Mongol Empire (Buell, 2012). In passing it should be said that scientific research in
bio-molecular archaeology puts it beyond reasonable doubt now that medieval plague
epidemics, both the sixth-century plague of Justinian (Sallares, 2007) and the Black
Death in 1348 (Campbell, 2011), were indeed caused by Yersinia pestis, the pathogen
of plague (Benedictow, 2010; Little, 2011; Tran et al., 2011; Harbeck et al., 2013).
Plague itself travelled inside infected fleas, its vector, which were carried in cargoes of
grain and commodities such as textiles, and perhaps also sometimes in humans carry-
ing asymptomatic infections. Plague also required a rodent as a host, since humans are
largely incidental to plague. As its native rodent hosts in east Asia do not occur in the
Mediterranean, plague could not spread to Europe until after the prior dispersal of a
new host, the black rat (Rattus rattus) from India (Aplin et al., 2011), a process
which probably commenced during trade between ancient Mesopotamia and
the Bronze Age Indus Valley civilization and continued with the classical spice
trade between India and Egypt, where the Plague of Justinian commenced in 541 ce.
Once it had reached the west, plague undoubtedly continued to spread along major
trade routes, for example the route of the early Byzantine grain trade from Egypt to
Constantinople, up the Tiber river to Rome, and along river valleys in France (Biraben,
1997; McCormick, 2003).
The spread of plague, just like the spread of malaria and other mosquito-borne
diseases, illustrates a very important general principle about the nature of vector-
borne diseases: namely that although a vector like a flea or mosquito is generally
required for disease transmission to individual human cases, nevertheless long- distance
movement of such diseases requires the long-distance movement of humans because in
general the vectors and other hosts will not move very far on their own. (The most
striking instance of long-distance movement of diseases caused by long-distance
human migrations is the Polynesian migrations in the Pacific in prehistory (Sallares,
2010), but this strays too far from the topic of this chapter.) Most mosquitoes, for
example, rarely travel more than a few hundred yards from their breeding sites (unless
they fly onto a ship, or an airplane today), while the black rat is not a migratory crea-
ture (unless it too goes on board a ship), and fleas are obviously even less mobile. That