Aru Gupta, Ph.D.
August 2026
(5 Minutes)

Picture this: You come across a beautiful stone henge on a meadow in the middle of nowhere. You gently touch the rocks, perhaps imagining ancient hands that erected the stones…and you find yourself in medieval Scotland. You’re a time traveler. You’re also almost certainly about to die in a few weeks.

We think of the Black Death, also known as the bubonic plague, whenever we talk about medieval Europe, because it destroyed nearly a third of Europe’s population. And while cyclic instances of the plague regularly decimated Europe, there are many more pathogens lurking in the recent past that could quickly kill you just as effectively.

The question of what would actually kill a modern human dropped into the deep past is more biologically interesting than most fantasy or science fiction lets on. Your immune system is like a library of every pathogen you survived. Every vaccination you’ve received, every childhood cold, every stomach bug you shook off has trained an elaborate defense network. You carry antibodies, memory B- and T-cells, and a gut microbiome shaped by centuries of co-evolution with specific microorganisms.

I suppose you have one comfort going backwards in time—many ancient diseases never really disappeared, so you’re not completely defenseless. But here’s the catch: which pathogens you’ve been exposed to matters enormously. You have no immunity to pathogens that simply no longer exist. Take smallpox. Eradicated via extensive vaccination in 1980 and celebrated as one of medicine’s greatest triumphs, it also means that routine vaccination stopped decades ago. Virtually no one alive today carries any immunity to smallpox whatsoever. If you materialized in an era where smallpox was actively circulating, which covers most of recorded human history, you would be as immunologically naive as an infant, facing one of the most lethal pathogens Homo sapiens have ever encountered.

The only killers weren’t exotic ancient poxes or plagues, though. Pre-modern water supplies were biologically rich in ways that would overwhelm a modern gut with no prior exposure. The concentration and diversity of various gut pathogens like Vibrio cholerae, various Salmonella strains, Cryptosporidium, or Shigella in any ancient water source would hit an unprepared intestinal microbiome like a sledgehammer. Severe dehydration from untreated diarrheal illness like dysentery could kill a healthy adult in days. Additionally, ancient and medieval humans coexisted with intestinal worms and other parasites at levels that are genuinely difficult for modern people to conceptualize. If the dysentery didn’t get you, parasite-induced malnutrition would.

Tuberculosis would pose a slower but equally serious threat. TB has circulated in human populations for thousands of years, flourishing in the crowded, poorly ventilated settlements that characterized most of ancient and medieval urban life. Without treatment, it becomes a chronic wasting disease, progressively destroying lung tissue over months or years. Today we treat it with antibiotics. In the past, you simply deteriorated from consumption.

And don’t underestimate the mundane. A cut from a rusted nail in a medieval marketplace, or a scrape from a fall on cobble stones covered in animal feces, could spiral into sepsis. If you received the Tdap vaccine (for tetanus, diphtheria, and pertussis) before traveling to the past, you would probably be protected from lockjaw (tetanus) for a few years, but a random bacterial infection could do you in. Before antibiotics were discovered, pneumonia, tooth infections, and skin abscesses routinely killed otherwise healthy adults. The lethality wasn’t in the drama of exotic disease; it was in the total absence of any backstop once infection took hold.

If you somehow managed to bring modern antibiotics with you, you would most likely survive a bacterial infection, but I wouldn’t say the same about any probiotic microbes you might acquire in the “past”. Ancestral gut microbiomes had higher diversity, including unknown species, and fewer antibiotic-resistance genes compared to modern industrialized populations. One course of antibiotics would wipe them out. Antibiotic resistance is a modern problem, a product of decades of widespread antibiotic use in medicine and agriculture driving evolutionary pressure on bacteria. Ancient bacterial strains —the Staphylococcus, the Streptococcus, the Mycobacterium tuberculosis of four hundred years ago —had never encountered these drugs. A standard course of penicillin in medieval Scotland would be tremendously effective. You’d survive the bug, but then you’d probably be executed for being a witch.

In all my previous points, we have assumed that the past is the aggressor. But the most unsettling dimension of the scenario runs the other direction entirely. Your body is a living archive of modern pathogens. Every bacteria, virus, and microbial hitchhiker that you’ve ever encountered is a terror that ancient populations had never encountered and had no immune memory of. You wouldn’t need to be visibly sick to be dangerous. Your casual nasal flora, your gut bacteria, the rhinovirus you picked up two weeks ago and shook off in a day—any of these could be catastrophic to immune systems that had never seen anything like them.

The “virgin soil epidemics” that occurred following European contact with the Americas introduced smallpox, measles, and influenza to Indigenous populations with zero prior exposure, is well-documented history. These newly introduced pathogens killed millions, with mortality estimates in some communities reaching 50–90%. The diseases weren’t unusually virulent by European standards; they were simply unfamiliar to the Indigenous American immune system. So, if you did travel back in time, would a past bug get you before a future bug got them? 

Life before modern medicine, sanitation, and public health was a series of gambles with deadly infections. If I haven’t yet convinced you that time travel would be deadly rather than romantic, and you do go looking for stone henges that transport you to some pocket of history, drink nothing, touch no one, and try to get back to the present as quickly as you can. 

Further Reading:
http://www.kingjames1ofscotland.co.uk/publications/epidemics-james-scotland/
https://www.nature.com/articles/s41586-021-03532-0
https://hms.harvard.edu/news/guts-our-ancestors
https://www.sciencedirect.com/science/article/pii/S0047248414002620?via%3Dihub