If germs from the past was brought to the present/future on a person (or by some other means of transportation) and we have immunity to its descendants, like the modern germs for example, would we be immune to the germs brought to the present? I have only seen conversations about people bringing germs from the modern day to the past, but what happens when we flip the tables.
You don't know how actual your question is: link. The melting of the polar cap might actually release long dormant viruses, bacteria and funghi.
In August 2016, in a remote corner of Siberian tundra called the Yamal Peninsula in the Arctic Circle, a 12-year-old boy died and at least twenty people were hospitalised after being infected by anthrax.
The theory is that, over 75 years ago, a reindeer infected with anthrax died and its frozen carcass became trapped under a layer of frozen soil, known as permafrost. There it stayed until a heatwave in the summer of 2016, when the permafrost thawed.
This exposed the reindeer corpse and released infectious anthrax into nearby water and soil, and then into the food supply. More than 2,000 reindeer grazing nearby became infected, which then led to the small number of human cases.
About the risks
"Following our work and that of others, there is now a non-zero probability that pathogenic microbes could be revived, and infect us," says Claverie. "How likely that is is not known, but it's a possibility. It could be bacteria that are curable with antibiotics, or resistant bacteria, or a virus. If the pathogen hasn't been in contact with humans for a long time, then our immune system would not be prepared. So yes, that could be dangerous."
We tend to correlate the word "evolution" with the word "improvement", but that is a false association. "Evolution" is more closely related to the word "change".
If at a particular moment in the past, a disease emerges to threaten the survival of a population, then the portion of that population which had previously "evolved/changed" to resist that disease would have a higher survival rate than the rest of the population. Then for a time, that higher survival rate would give them a propagation advantage, allowing their particular genetics to pervade a larger percentage of the population.
Once the presence of that disease decreased, the propagation advantage would fade as well. Other threats would emerge to endanger the population and other genetic changes would gain the advantage.
It is entirely possible over deep time that other genetic changes might break the historic disease immunity change in pursuit of immunity to a different threat. It is also possible that over deep time, an immunity change (which is no longer valuable because it's disease is not present) can disperse or even vanish from a local gene pool. Gene lines die out all the time for no reason greater than personal dating choices.
So yes, a historic disease can be a threat to a modern populace even if they had survived the disease in their distant past. As with wisdom and un-persisted knowledge, we tend to loose what we do not use. So if a particular version of a disease has been gone for a long enough time, we may very well be susceptible to it again.
This is an something that has actually been tested in the long term e-coli experiment. In bacteria separated by several generation older generations can outcompete there distant descendendants. It is not a given of course but the chances are not insignificant either. This is because the population of bacteria is constantly changing as bacteria adapt to each other and their hosts but such adaptations are not immediately lost nor always useful in the long run. For example an ancestor may have produced a toxin to help kill off competing bacteria but it may completely kill off that competing strain or other bacteria may become resistant to the toxin, so later it loses the ability to produce the toxin since the toxin takes calories to make. But put the toxin producing ancestor in with its descendants and it may wipe out is later descendants as well.
There is also an effect with diseases that causes them to become less lethal, later strains of HIV are milder for instance because living hosts are more likely to spread the disease. A reintroduction of the original strain would be devastating since it would be more virulent and kill a lot of people even as it drives itself extinct. This is not uncommon, very virulent diseases are worse off evolutionarily than less virulent descendants precisely because they are much worse for the hosts, and living hosts are better at spreading disease.
Now the but, if you go back far enough the chances of catching a compatible disease are lessened, simply because there is nothing with similar biochemistry to you around. For instance go back to the carboniferous and there are no warm blooded animals around and the chances of a comparable disease are extremely low. The worst diseases tend to be from your own species or closely related species, that or you need long contact with the species like humans and livestock.