Christmas, as we all know, is an opportunity for people with differing immunity systems to expose each other to new and exotic contagions. People will travel hundreds of miles, in fact, to do the hard work to spread these bacterial and viral entities, who, confined to small communities, must sometimes feel like they are beating their little heads against an immunity wall. They need a vacation, too, and so love nothing better than to pile into the car, air- and mucous-borne, and visit Aunt Tilly, Uncle Bob, and their tabula rasa brood. There, surrounding a tree and table full of Christmas cheer, the unwitting carriers can exchange their home-grown parasites happily.The sneeze, of course, is a contagion's most dramatic reproductive mechanism. A sneeze can create a maelstrom of malevolence in one orgasmic detonation. But a simple handshake or kiss can just as easily do the trick. Human-to-human contact is not the only means of transport; animals--wild, domesticated, and semi-domesticated--are excellent sources of illness. The parasites (fleas, ticks, lice, etc.) that live on these animals can be even more prolific. And we must not forget nature's little workhorses, the fly and the mosquito. Regardless of the origin, however, the most efficient large-scale dissemination of a virus is human travel. History is the saga of clashes between peoples with disparate immunity systems. Any sedentary civilization is bound to fall prey to a visitor's cornucopious menu of infectious bacterial and viral disease germs. And visa-versa, of course. Visitors may come in peace or in aggression, but either way, it's viral and biological warfare. An inferior opponent's best defense may be their own wretched physical condition. No less a military strategist than Napoleon himself was mere putty in the hands of dysentery and typhus. Hans Zinsser, in his 1934 ground-breaking study of disease and culture Rats, Lice, and History, states that "It is hardly debatable that the power of Napoleon in Europe was broken by disease more effectively than by military opposition." Throughout history, aggression was not the only means of a contagion courier. An ostensibly peaceful mechanism such as trade was a perfectly adequate tactic. As the Old World expanded via these land- and water-traveling merchants, citizens of Asia, Europe, Africa, and the Middle East enjoyed centuries of exposure to exotic epidemics. Over the course of time, however, resistances to these many diseases developed, and the numbers of casualties slowly decreased. When the Old World met the New World, via trans-oceanic travel, the result was catastrophic. The New World inhabitants, a "hitherto virgin host population," as eminent historian William H. McNeill would call them, were in no shape to meet the Europeans. The natives of the so-called American continent sported an immunity system far less worldly than the manifest destiny men who came a-calling. The explorers, Cortez, Columbus et al, not only courted the New World peoples with human-to-human infection, they doubtless had stowed-away in their ships all sorts of animals and insects, brimming with infectious goo. Thus, the Old World visitors were teeming pustules of death and disease. The Welcome Wagon, if there was any, was on the front line of susceptibility. Smallpox was particularly destructive to the "virgin host population." Old and New Worlders would have all considered epi- and pandemic disease as punishment from a divine entity. Nevertheless, there surely must have been enormous distrust of the invaders. Perhaps racism's root has in no small part something to do with this distrust. It's obvious: differently colored peoples--differently-colored from you--contain exotic contagion. The holiday of Christmas might seem to some to be its own kind of disease. Certainly, over the years, it has spread like a plague to encompass all of December and nearly half of November. Consumerism appears to be contagious, as the bright and shiny viruses of advertising work their way into our systems. Each year we struggle with and against the materialism of Christmas, trying to focus on the heart of the holiday. Prosperity might be a part of it, but family and friends are at the center of Christmas's ruminations. Most of all, though, if we are lucky enough to have it, we treasure our health, recognizing that it's our single, greatest gift. And so as we delight in our own gathering around the Christmas tree and table, as we eat and are eaten by our parasites, let us give thanks: to antibiotics, to inoculations of all kinds, to condoms that prevent the spread of AIDS and other STDs, to education that informs us about prevention techniques, to soap, and to relatively stable epidemiologies. Given the history of diseases's scourge of humanity, that's a heaping handful of thanks, a bounty of gifts.