It could make a good Great Filter candidate.
Didn't neanderthal disappear during the last climate warming?
And now we're the only intelligent species left..so
The great filter argument is that there is no intelligent life in the universe because as soon as it gets intelligent enough to dominate, "fate" wipes it out. Arguing that fossil diseases can fulfill this task is to argue against evolution and technology - these are diseases that didn't wipe us out backintheday and now that we have several hundred years of epidemiology under our belts, it's unlikely that we'll suffer at the hands of a bug that petered out. The two human diseases mentioned - smallpox and anthrax - are also mentioned in the place that has done the most weaponization of both: Russia. Smallpox vaccine is over two hundred years old by now and some tens or hundreds of thousands of healthcare workers and soldiers are already vaccinated. Anthrax, for its part, is perhaps the most domesticated bioweapon in anyone's arsenal and regardless of the thawing permafrost, this is permafrost we're talking about: no one is concerned about a sudden outbreak in Edmonton, they're worried about something drifting downriver from the hinterlands. It's a vexing problem to be sure, and another ancillary problem caused by global warming, but if I were to pick "likely sources of global pandemics for $500" alex, "permafrost" wouldn't be high on my list.
I'm curious now - does the age of these pathogens affect modern humans' susceptibility to them? I wonder if there's some sort of function where, as you move back in time, diseases are easier to fight off because our immune systems recognize them and their offspring, however after a certain point the old disease is different enough from its modern counterpart that it's basically new to us all over again. Think like the great x 1000 grandparent of the modern strain of rhinovirus. Would/can our immune systems make that connection and fight accordingly?
The problem is, evolution and epidemiology collide/coincide on a different basis than you'd think, from a self-centered humanitarian standpoint. Richard Preston made the point in The Dead Zone that really, Ebola is a terrible failure of a disease - it's most advantageous to the organism if it can spread and multiply without injuring its host badly enough to provoke shunning and quarantine. You look at the common cold, which can be caused by a startling menagerie of viruses, and you see a large cohort of successful organisms that spread ambitiously but do not impact their hosts strongly enough to cause any prevention. You look at rabies and you see an organism so virulent that all other hosts avoid the infected. But evolution is a spontaneous process involving mutation good and bad and sometimes the benign pops up and kills the shit out of everyone. The Spanish Flu of 1918, for example. It wasn't just that it was a particularly nasty strain of influenza, it was also a bad circumstance for survival - lots of stressed out soldiers cramped in military hospitals, lots of travel with a bare modicum of quarantine procedure in place, etc. Hantavirus cropped up and killed twelve people back in '93. It generally doesn't kill anyone - I grew up with plague and by the time I was in high school it had killed two classmates so we never learned shit about hantavirus. But apparently the Navajos had dealt with outbreaks for a hundred years or more. Compare and contrast: sweating sickness killed you fucking dead within hours and that was probably hantavirus, too. But we don't even know that. When Preston wrote Hot Zone back in '93, we thought Marburg and Ebola were the same virus. Now we don't. We think Marburg came out of a cave. But we don't know for sure. So. Does the age of a pathogen affect our susceptibility to them? Probably yes, probably no, probably maybe. Epidemiology is a live science. Shit changes a lot.