Orsdxy

In my story, something pretty insane happens during the 3rd season.

The common conception was that an asteroid strike contributed to the extinction to dinosaurs, but unbeknownst to all it was not a giant impact that killed them off; it was actually a world wide plague that destroyed them.

The plague has now returned in modern day. The plague spreads world wide and is deadly for humans, though certain races are more immune than others.

My question is this; what is a scientifically realistic way this could happen? If a plague did indeed kill the dinosaurs how could the plague:

A) Become stagnant for millions of years and disappear from history?

B) Only to return again in modern day to wreck havoc for a second time. Whether it be by the hands of a scientist or through some other event. It returns and hits hard.

C) Be deadly to humans despite being tens of millions of years old?

Trivia

Simple facts about the plague:

• It is only deadly for certain living things, unfortunately humans
  make the list.


• It affects cell motility.


• Roughly 30% of individuals have some degree of immunity towards it with race playing a factor. No one has complete immunity (at least not naturally).


• It spreads fast.

Note: While I was originally only looking for hard science answers, I am now accepting any explanations that are at least within the realm of realism. Hard science answers are still welcomed though if anyone has anything to contribute. But I've come to understand that the nature of my question makes hard science quite limited.

Plague can lie dormant for years... centuries... millennia...

From the CDC we learn (emphasis mine):

The etiologic agent of plague, Yersinia pestis, is a gram-negative coccobacillus and a facultative intracellular pathogen. Y. pestis exhibited the highest overall mortality rate of any infectious disease from its earliest recorded emergence through 1941. During 2010–2015, a mean of 650 cases were reported globally each year, with a case fatality rate of 23%–41% (depending on manifestation as bubonic, pneumonic, or septicemic plague), rising to 66%–100% when adequate medical care was not promptly received. Y. pestis primarily infects small ground-dwelling mammals, specifically of the taxonomic order Rodentia, but maintains high spillover potential to other vertebrates, including humans, caused by its high virulence and fleaborne transmission. Epizootic plague is typically vectored by multiple flea species and is transmitted within and between meta-populations of hosts by flea bites.

Plague ecology is characterized by sporadic epizootics, followed by 2–5-year cryptic dormancy periods. Despite much information on epizootic transmission mechanisms, little is known about the origin of re-emergent plague cases in wild animal populations. Plague among wild animals commonly re-emerges in plague foci after multiple years of inactivity, despite ongoing biosurveillance and attempts at detection during interepizootic periods. The existence of environmental plague reservoirs has been theorized for >80 years. Various avenues of recent research suggest that soil-dwelling amebae may be competent environmental reservoirs of Y. pestis. Amebae are a taxonomically diverse group of phagocytic organisms residing in every major lineage of eukaryotes. Amebae are pervasive in soil and water environments and are recognized for their ability to harbor pathogens that drastically affect ecologic communities. Free-living amebae cycle between 2 distinct life-states: trophozoites, an active, mobile, feeding state; and cysts or spores, a robust dormant state induced in part by adverse environmental conditions.

That's a long-winded and technically precise way of saying that while the Black Death was spread via rats and fleas — the problem is that it lays dormant in soil and water, waiting for the right combination of climate and ecology to become active again. This is why it keeps flaring up all over the world.

It is reasonable and believable that the pathogen that killed the dinosaurs in your story, a pathogen that would have flourished in predominantly cold-blooded critters living in a Mesozoic climate (and not being dissimilar to Y. pestis) is waiting for a big old lizard and the same climate to coincide again. It's in the soil. It's in the water. We've just never had a reason to look for it.

But, lizards ain't humans

We're missing something, though. We need to jump the blood-brain barrier from cold-blooded lizards to warm-blooded primates. I give you: Salmonella.

Salmonella are commonly found in all types of reptiles and can spread from reptiles to humans when something contaminated with reptile faeces is placed in the mouth. (Source)

So, Lizard meets warm, moist, somewhat old-fashioned climate, voodoo plague rears its ugly head and bonds with the salmonella... lizard poops right on top of this amazing Cacao plant that happens to benefit from the Mesozoic climate, bean is picked and (say it ain't so!) not cleaned very well... and served as your favorite chocolate confection at Walmart.

And a week later 90% of humans are zombies.

I think that is pretty straight forward: A plague that is that deadly for the dinosaurs is a huge evolutionary pressure. Therefore most dinosaur species died out, but the line that later became birds had some key mutations so that the plague was no longer deadly to them, but merely a nuisance (such as a cold for humans). Note that there is evolutionary pressure on the plague as well, not to be too deadly, therefore there are examples that deadly diseases get less virulent over time.

At some point, the plague crosses the species barrier and wreaks havoc among humans.

This is also not unheard of. Ebola seems to be an equivalent of a common cold among flying foxes who are adapted to it. It is quite deadly for humans.

This scenario is entirely plausible. I just don‘t think that the plague would actially kill all the dinosaurs because of the evolutionary pressure to not kill its hosts too fast. But possible: Yes.

An environmentally linked bacteria.

The bacteria needs to be present but harmless. What people don't know is that the bacteria has environmental triggers linked to the CO2 levels in the air which cause it change and produce toxins, a bit like algae.

See Harmful Algae

Currently man is producing CO2, pushing it to levels not seen since 50 million years ago so you could in theory hit the same environmental trigger.