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even greater horror of epidemic disease. In their already weakened condition, they had few
resources to fight it.
ONE LOUSE, TWO LICE
As any parent of schoolchildren knows, the louse is always with us. Indeed, the shared history
of humans and lice constitutes a remarkable instance of co-evolution. 19 Five million years ago,
when the ancestors of modern human beings diverged from the chimpanzee, a new species of
lice ( pediculus humanus ) joined us on our evolutionary adventure. A second historically mo-
numental divergence occurred when human beings began to wear clothing. Ambitious lice
migrated to the human body ( pediculus humanus humanus ), where they developed the nifty
expedient of depositing their eggs in human clothes and taking up residence there. Striking
evidence of our close, co-evolutionary relation with the louse is its slim genetic signature,
characterized by a deficit of genes associated with environmental sensing and precious little
metabolic engineering. Thedomesticated louse hasnoneed forreceptors ofsmell ortaste, and
possesses the smallest number of detoxification enzymes of any insect. Why wander about,
when the human body and its vestments offer the coziest possible accomodation? 20
It is unclear exactly how and when the modern typhus bacterium ( rickettsia prowazekii )
emerged to fatally complicate the host-parasite relation between humans and lice. Some ar-
gue that the famous plague of Athens described by the historian Thucydides was in fact
typhus. More likely, however, is that typhus resulted from the chaotic biological exchange
initiated by European colonization of the Americas in the fifteenth century. One convention-
al hypothesis traces the typhus pathogen from the Far East to Spain in the fifteenth century
and from there to the Americas and a global imprint. But the recent discovery of a nonhuman
reservoir for typhus in the American flying squirrel suggests an inverted etiology, namely
that—like yellow fever, cholera, and syphilis—typhus originated as a colonial disease brought
back to Europe from the New World. The deadly typhus, according to this scenario, “was born
in the chance meeting of an American rickettsia and a Spanish louse.”
The impact on modern human history of this freak biological union is incalculable. Typhus
decimated Napoleon's army on his disastrous retreat from Moscow. A century later, during
the revolutionary period 1917-25, twenty-five million Russians contracted typhus, killing an
estimated three million. More recently, in the 1990s, typhus reasserted itself as a major glob-
al threat, infecting over one hundred thousand people during the civil war in Burundi. In
terms of sheer numbers of victims, louse-borne typhus has not only been decisive in the out-
come of major military conflicts during the last five hundred years, it has in that time “killed
more people than all conflicts combined.” 21 The typhus epidemic that swept across peacetime
Europe in 1817-18 is thus but one episode in a five-hundred-year biological disaster: an ex-
ample of the vulnerability of human communities to modern, globalized pathogens in times
of material distress, be it war, famine, or an ecological breakdown precipitated by climate
change.
In one sense, the louse has gotten a bad rap in all this. After all, it is we, as the natural
reservoir of the typhus rickettsia, who first infect it . The louse ingests the disease bacter-
ia from the blood of its human host, the rickettsia carrier, whereupon it is mortally infec-
ted. Between infection and death, however, lies the window of opportunity for the louse to
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