This question is in direct response to the early comments on my last question: Time traveler has identified a unknown epidemic is likely spread by mosquitos. How much can he help combat it?
The comment suggested that a time traveler traveling back in time would bring back disease that would threaten the people of the past. The analogy to the colonization of the Americas and the literal decimation of the American people to disease was brought up.
My belief is that a time traveler from a modern first world nation is not in risk of trigger a "New World scenario" of epidemics; However, I'm basing this on my understanding of evolution only, I'm not an expert in medicine specifically. Thus I'm not confident enough of my own medical knowledge to say for certain, and so am asking the question in hopes of getting feedback from other's who are better qualified to speak to the risk.
My reason for thinking this is a lower threat is because I think modern medicine/sanitation has taken the place of evolution in combating disease, with a result being that the germs a modern 'health' first world individual carries likely not being viral enough to threaten a foreign population.
In the colonization of the New World those colonizing were carriers for a number of extremely deadly diseases, like small poxs, which had acted already spread through europe and killed a large percentage of the population in plauges of the past. Thus the ones still alive had evolved a strong immunity to these diseases (the only reason they survived the plauges) and thus could safely be carriers for viral diseases without triggering new epidemics in Europe. That meant those colonizing America were carriers for numerous viral disease that had already killed large swaths of Europe before evolution kicked in.
I believe that modern man is not a carrier for these sort of viral diseases. My reasoning is that we now combat potential epidemics with medication and vacination, as well as basic sanitation. Thus I feel like we are stopping the diseases before they can kill large portions of the population. That means we aren't evolving immunity to viral diseases as Europeans of the past did, but killing all that lacked the immunity. There is no longer a significant evolutionary pressure on developing immunities because of modern medicine and thus modern man isn't likely to have a significantly higher evolved immunity to modern germs as someone from the past who wasn't exposed at all.
To back this up I would point to the fact that Europeans colonizing the America's didn't catch many diseases from the Americans (syphilis being the key exception), because the American people were not carriers of viral epidemic diseases the way the Europeans were.
I also believe that old epidemic disease, like the black death, which modern man would still have an evolutionary advantage against (since their ancestors were forced to evolve a resistance to them to survive) and thus theoretically could be carriers for today would have likely mutated over the centuries to the point that any modern decedents of the disease would no longer be a life threatening epidemic capable disease.
So, is my theory correct, or is it flawed reasoning? How likely is it that modern man is carrier for a disease viral enough to threaten the population of people from the past?
edit: I don't think the linked question is a dupicate. It focuses on antibiotic resistance, which I don't care about, and on the idea of rather a disease is capable of wiping out all of humanity.
I'm curious about normal, regardless of rather it's antibiotic resistant, diseases which would be viral enough to spread and dangerous enough to kill infect people from the past, rather or not they can wipe out all of humanity. I know, as many answers to the linked question state, that any disease bright back wouldn't threaten all of humanity because some will be resistant. I would like to know if there is a risk of bringing back a disease which would still spread and cause noticeable harm via regular infectious route even if it isn't close to extinction level event.
Furthermore I'm asking about people of the past! they lack the sanitation and medical knowledge of the present, and thus would be more at risk for a normal, potentially lethal, disease spreading if a time traveler did bring one back then modern man, as they wouldn't know how to quarantine and fight it through other methods beyond just antibiotics.
