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Disease & time travel
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Time-traveling back to the middle ages has seemingly always been a popular theme in kids' shows, science-fiction books, etc. But what would actually happen if a person from our era traveled back in time? How would the difference in air pollution make an impact on the traveling person, and what medieval diseases would she get? How many of the people there would die of bacteria that the modern person brought with her?

—Kid from Sweden

Limiting the discussion to time-travel destinations predating the discovery of antibiotics and vaccines, we find there are two main types of health-related trouble the intrepid temporal explorer could be setting herself up for.

The first possibility is falling prey to ancient diseases. Life during the middle ages, and pretty much any other era until recently, was incredibly dirty, and depending on time and place, clean food and water were more or less unknown. Air pollution could be a significant hazard if you traveled back to Victorian London, or for that matter spent a lot of time indoors around a smoky dung fire—evidence of lung disease has been found in ancient societies ranging from Egypt to the U.S. Southwest.

Water and food contamination from lead dishes and cups might be a problem, although that would require lengthy exposure. Simply eating or drinking anything prepared before about 1900 would present more immediate risks—our time traveler would have a fair chance of intestinal worms, trichinosis, giardia, or other parasites too numerous to list. Anthrax, tuberculosis, and botulism can all be spread by eating the flesh of infected animals.

In early urbanized areas, dysentery, cholera, and typhus were the rule rather than the exception. For most of human history it would be difficult for our traveler to avoid smallpox, cowpox, and variations of influenza unknown to modern times; lepers and plague victims would warrant a wide berth. It's true that the modern suite of inoculations would likely protect our time traveler from many common diseases.

And of course food variety and balanced diets weren't the norm for most of human history. Nutritional diseases such as scurvy, pellagra, and goiter could well afflict our traveler if, having dodged all the above, she were obliged to subsist long enough on local cuisine.

The other, far worse possibility is visiting modern plagues on the past. New diseases have shown up unpredictably throughout history. In 1967 the U.S. Surgeon General boasted we'd won the war against infectious diseases; less than a generation later HIV/AIDS emerged. If our time traveler was a temporal Typhoid Mary, she might gift the past with such latter-day scourges as severe acute respiratory system (SARS), which in the first year of its appearance caused nearly 10 percent mortality even with modern medical care; Ebola and Marburg viruses, although victims die so quickly the spread might be limited; and of course HIV/AIDS, with a current worldwide toll of 30 million dead.

But new diseases wouldn't necessarily be the gravest threat. Possibly the real danger would come from ordinary illnesses that had evolved significantly over the centuries, in part because weaker strains had been killed off by antibiotics. Keeping Doctor Who's peregrinations in mind, remember also that time travel could involve journeying through space as well, meaning one could unwittingly bring pathogens to regions with minimal resistance.

The result might be an epidemiological catastrophe rivaling those that actually occurred. As is now well known, Old World diseases virtually depopulated the Americas within a short time after first contact, with estimates of mortality ranging as high as 90 percent. Fatal bugs included not just smallpox, which alone may have killed more than 15 million people following its introduction in Mexico in 1520, but everyday afflictions such as measles, mumps, chickenpox, and scarlet fever. And let's not forget the 1918 pandemic involving a newly mutated strain of influenza that killed 50 million or more.

In his classic War of the Worlds, H.G. Wells imagined that invaders from an advanced civilization might be wiped out by microbes harbored by us primitives. Judging from history, aspiring sci-fi authors might want to note, the more likely scenario puts the casualty count the other way around. cs