252 robert sallares
bio- climatic zone to the south of the Mediterranean. There certainly were some
interesting contacts across the Sahara in historical times that are relevant to disease
history. For example, the movement of black African slaves in antiquity and especially
the early medieval period, following the spread of Islam across the Sahara desert,
brought the sickle-cell trait, a human genetic mutation that gives resistance to falcipa-
rum malaria, from west Africa to the Mediterranean, where it is found in some human
populations today (Sallares, 2002: 39). However, as we shall see later, there are very
significant differences in the transmission patterns of malaria in sub-Saharan Africa
which mean that the demography and effects of malaria on human populations there
today are in many respects quite different from the effects which it had on
Mediterranean human populations in the past.
A focus on malaria and leishmaniasis enables us to begin to define a community
of pathogens in the Mediterranean in the past with some significant features which
differentiated it from the communities of pathogens to the north, in northern
Europe, and to the south, in tropical Africa, and so to establish the Mediterranean
as a useful category of analysis in disease history. Of course the composition of the
Mediterranean pathogen community in the past was not determined solely by cli-
mate; it is clear that the Mediterranean climate regions in California, Chile, South
Africa, and Australia had pathogen communities that were quite different from
those around the Mediterranean Sea and from each other before Columbus and the
expansion of Europe, which led to the microbial unification of the world, to use the
phrase of Emmanuel Le Roy Ladurie (1981: 83). Evidently historical contingency
played a large part in the formation of the Mediterranean pathogen community, but
this merely serves to emphasize the distinctiveness of the lands around the
Mediterranean Sea.
A gradual increase in connections between different regions, leading to the spread
of both pathogens and vectors, played a key role in shaping the pathogen community
of the Mediterranean throughout history. The idea of mapping the spread of diseases
is a very important component of medical history (Koch, 2011). The history of dis-
eases cannot be understood without considering the movements of armies, merchants,
craftsmen, slaves, pilgrims, and other travelers. This is why the history of diseases
should not be divorced from other branches of history, even if making sense of it
requires technical knowledge which many historians may not possess. Increasing con-
nectivity is still very much an ongoing process today as far as diseases are concerned.
The contemporary example is the spread of the tiger mosquito, Aedes albopictus,
around the world on board merchant ships from its original home in south-east Asia,
bringing dengue fever and chikungunya, virus diseases of south-east Asia and the
northern shores of the Indian Ocean, to Mediterranean Europe. The tiger mosquito
has become endemic in Rome, for example, by laying diapause eggs in the late autumn
with delayed hatching, solving the problem for a tropical organism of surviving south-
ern-European winters (Toma et al., 2002). The first outbreak of chikungunya in Italy
was in two villages near Ravenna in 2007 (Rezza et al., 2007), while cases of autoch-
thonous transmission of both chikungunya and dengue fever had occurred in
south-eastern France by 2010 (Gould et al., 2010). It has been shown that the tiger
mosquito is more likely to spread along shipping routes today where there is a high
volume of traffic than along routes where there is a low volume of traffic; the spread
of this disease vector is directly linked to the volume of trade (Tatem et al., 2006).