Although I think this question seems to be out of scope of this group, I'd try to answer to make it more fit.
If you are interested in genealogy, and ever had created your family tree, you should have noticed that your ancestor had more children than we do (at least in Western culture). Having 10 children was nothing unusual. If you look deeper, you will notice many of them died being infants, so not so many survived to their adulthood.
Of course the reason is poor condition of past medicine. The development allowed us to live longer, healthier (not only medicine counts, but eg. washing your hands before eating), and thus happier (also because it's easier to get a car or brand-new smartphone).
However, the theory of evolution (in simple) says that the weaker organisms shall die, while stronger will survive and multiply. The theory says it's not good to say "weaker/stronger" or "worse/better". The organism that fits better its environment, is better, but we can't say a crocodile is worse than an elephant only because an elephant is a mammal. They are equal, because they live. If a crocodile is not fast enough to hunt an antelope, it will starve to death. If an antelope is not fast enough to escape, it will be eaten. If they die, they won't multiply and bad genotype will be lost.
The same applies to humans, of course. We could harness the nature using weapons (against bigger animals), tools (against flora) and medicine (against smaller organisms like bacteria or parasites).
The development of religion and ethics made us think that human life is always worth to protect.
This is of course against the evolution theory which says that only good organism "is" protected and the worse should be (I'm sorry) eliminated. The development of medicine made us weaker than people from the past, because bad genotypes were stored because we've invented penicillin, we managed to fight some diseases.
We are less immune if compared not to all people from the past, but to all from the past who managed to reach their adulthood and have children. Probably, statistically, if we take all people who lived then (also those who died young) and compare with all living now, we are healthier. But shouldn't we compare only those who survived?
The development of antibiotics allowed to kill many bacteria, but those which survived, are stronger. They need to be killed with stronger antibiotics, but this also weakens us.
You may also notice, when talking with older people, that they managed to live in stronger climate (eg. colder winters), and they are doing pretty well now, while we have civilization diseases like allergies. We are not able to climb a mountain that 80 yo. person travels twice a day.
Read this article, this one, this one, or this one (I understand some of them are popular science and should not be considered as a scientific source). Maybe you'll google more articles on the same topic.
So answering your question: there are many theories leading to a conclusion, that men from past would manage to live in our times better than we do.