From what I read, merely eating would be a considerable hazard for a hypothetical time traveler traveling beyond a few centuries in the past. Apparently, strict regulations in the processing of food (dating to the early 20th century after the release of Upton Sinclair's the Jungle) has considerably weakened a modern person's immunity system. In other words, most diets beyond the past hundred years would very dangerous to someone living today. As they were often cooked/prepared in conditions that would be now considered "unsanitary." For example, if a time traveler traveled back to medieval Europe or pre-Colombian Americas, the food there would at best make them considerably ill.
I remember reading stories where they had to take a series of shots for this reason. I think it was Andre Norton's Time Trader novels, where modern people dressed as Cro Magnons. They also had to be in excellent physical shape and learn the language and customs etc, spend a good deal of time living outdoors in skins and using bone and stone tools. That's if I remember right, but it's been decades since I read them.
You sort of see something similar today with 'Monteczuma's Revenge'. That's why I pack my lunch whenever I time travel.
I'd be interested in seeing some citations for this being categorically true in regards to most food. Say I went back in time to medieval Europe and stayed with a peasant family. If those peasants had grown some peas and carrots in their garden and then trapped a rabbit and made a stew, and got some fresh milk from a reasonably healthy cow, what about that is going to make me seriously ill? How would beans and corn tortillas be prepared in such a way as almost certainly to make me seriously ill?
It may not apply to plant based dishes, I was thinking a similar thing, that it may end up being a fresher diet than many are used to today. But animal proteins may not be cooked to the proper temperature. But, in an age without modern plumbing, if you think about how many dirty hands would handle the produce and utensils, etc. maybe any food could be a risk for someone from the present.
When I was recovering from gluten sensitivity/leaky gut syndrome, I did what's called a clean gut diet, consisting of only extremely 'clean' foods. No tomatoes or other nightshades, no beans, etc etc. It left a very short list of foods I could actually eat. It was extremely helpful and made me feel great, after a while there was no more inflammation, which now is said to be one of the major sources of disease in the body. But after a while I realized I was becoming like the Boy in the Plastic Bubble (anybody old enough to remember that John Travolta movie?)—he had no immune system so had to live in a totally 'clean' environment, anybody visiting him had to wear a plastic suit and breathing mask etc. He basically lived in a giant habitrail. If you get yourself used to an environment or food that's unnaturally clean, free of toxins, your natural immunity lessens. Your body develops the antibodies to fight off whatever your'e exposed to. So I realized after I had fully recovered that I'd better gradually ween myself off all the clean food, or as soon as I bite into a Big Mac or something I'll keel over and die. I'm proud to report I am now robust and healthy again, thanks to eating nice and dirty. Something like 80% of your immune system is in your gut, in the form of bacteria. And side note, it can only do its job properly when the gut isn't busy at its other job of digesting food. This is one of the reasons fasting is so powerful and health-promoting. Once your gut is empty of food it goes to work as an immune system. Or at least it can work far more effectively that way.
It basically works on the principle known as hormesis, or perhaps better called exposure therapy. It's the idea that you expose your body or your immune system or whatever to toxins in small controlled doses and you develop a tolerance or an immunity to them. It's the mechanism behind exercise (progressive resistance—breaking down muscle tissue so it grows back stronger, same as broken bones do) and behind why we develop a resistance to certain kinds of drugs or alcohol. It's also the concept behind rabies vaccination—they give you shots of spinal fluid from animals that have rabies to expose you to it in small doses. Gradually the doses get bigger, and voila—you have a robust resistance. There's probably a different process now, but this is how it was originally done, and it worked. I don't know how true this is or how well it worked if it is true, but I've heard of kings and princes etc taking small but ever-increasing doses of poison to develop a tolerance to it against assassination attempts. Or was that in fiction, I don't remember. A cool idea though.
Today we've become very overcivilized and pampered for the most part, living in climate controlled units with hot and cold running water and antibacterial soap and on and on—sleeping in soft beds, and most of us never really get outside and do anything strenuous. We don't acclimate ourselves to cold or heat or rain the way our ancestors did until very recently. We spend most of our time sitting, staring at screens large and small. But on the other hand, we've developed a powerful tolerance to certain toxins our ancestors never even heard of, like plastics and today's super-glutens and all kinds of chemicals. I doubt they'd fare well in our world either. It's a strange kind of evolution when you think about it.
I call hogwash. People live off the land all the time and slaughter, store, cook, and eat game in the wilderness. I think it would take some time for the average person who gets food from the supermarket, for their guts to adjust. Maybe some would get severely ill or die in the process, but that would be the exception I would expect.
People in the past weren't stupid. They will have observed - "if I touch this food when my hands are covered in crap, it will make me ill", and then they wouldn't do it. Likewise, people travel to countries with lower food standards all the time, with relatively few effects. The England cricket team used to get "Delhi belly" every time they toured India though.
I know it's true for certain regions, not sure if it would be true for time travel. If you're going to Africa or I think South America you need to take a series of shots over a period of time so you don't catch a whole range of diseases that would just kill you. When colonists started arriving in America they brought diseases that spread among the local natives and killed them in droves. At the time of course, with nothing like a germ theory, nobody knew it was going to happen or understood it.
It's the lack of refrigeration and improper temping that would get you. 40 to 140 F is the dangerzone where bacteria grows. 165 is the sweet spot where bacteria dies. Any "hot hold" items that drop to 139 have to be reheated to 165 for 30 seconds. Anything that remains in the dangerzone for longer than 4 hours must be discarded. That's why buffets make people sick. As for immunities? Not sure how often that would come into play, but aside from contamination, poor temping is almost always the culprit. Also anything that lives in the ground is highly susceptible to bacteria, most often from poor irrigation and livestock poop. That's why romaine lettuce gets recalled 4 or 5 times a year. We don't serve it at my restaurant for precisely that reason. ETA: also, contamination is more likely to occur from the modern processing system used today, where potential physical and chemical elements exist in the presence of food. That's why no glass is allowed in a kitchen, no rings are allowed on a chef's hand, and no chemicals can be stored above food products. ETA2: and don't get me started on PH and acidification. 4 is good. 4.1 can be mitigated. 4.6 must be discarded. Of course, this only applies to items that are eligible to remain in the dangerzone (40-140) for longer than four hours, which is why you need a HACCP from the department of health to serve sushi rice or sous vide food. And if you get health inspected without logs showing all of your temps and PHs, you will be in a world of shit. And for the love of God, people, if you ever have to fudge one of those logs, be sure to use different pens, different handwriting, and include mitigation procedures and food that you had to dispose of. In the real world, something needs to be tossed for a temp or PH violation at least 2-3 times a month.
They say that some people experience mild stomach upset when they switch to raw milk, but it settles down in a month or so. Me, I didn't experience that when I started drinking it several years ago. I think the biggest issue with our ancestors' food would be dodgy meat, beef and mutton hung a little too long and not refrigerated.
