Most animal species have multiple populations living in some degree of isolation from each other.   This isolation provides a buffering effect should some individuals become exposed to a contagious pathogen.   Even a highly contagious and deadly disease is likely to take out only the local population.   Until recently the Human race had the same type of protection.   The Bubonic Plague devasted Europe and Asia but never touched Africa, Australia or the New World.   Even as recently as 1918 the American Influenza epidemic had minimal impact on the rest of the world.   Now that has all changed.   People fly from continent to continent in increasingly large numbers.   Now a pathogen could travel the globe and reach every nation on Earth in a matter of days.   A disease as contagious as the cold and as deadly as Ebola would decimate the global population before we could mount a defense.   Presumably a few individauls would have a natural immunity and there are isolated populations even now.   Chances are humanity would survive but only a few percent would remain.   Our civilization would not.

Need an example?  Look how fast foot and mouth disease spread through the British Isles, then Europe, then to other continents.  And the instrument of the virus' spread...people.   And we only stopped that plague by killing and burning thousands of animals.   What would we have done if it was a human disease?   Set fire to the population of the UK?

So where would this pathogen come from?   There are two main sources; natural and man-made.   Viral strains can and will cross species boundries.   HIV is believed to be derived from SIV, the simian equivalent.   In turn the simian strain may have come from a feline strain, which may have been derived from a bovine strain.   Even more likely is that we encounter a bacterial pathogen which can generally infect a wider range of species than viruses.   We're constantly intruding into new areas of rainforest and other areas where man has not had a presence before.   When the early astronauts returned from space they were quarantined for fear of a space bug wiping us out.   Yet there are no quarantine procedures when we explore new places here on Earth where it's far more likely to encounter dangerous pathogens.  

If nature doesn't provide a plague we may well make one of our own, accidentally or on purpose.   We now have the technology to modify the genetic structure of plants and animals.   Do we know enough to completely predict the effects of those changes?   Not hardly.   Have we shown we can control the release of modified organisms?   Just the opposite.   Look what happened with Starlink, the genetically modified corn that was not approved for human consumption.   It quickly found its way into the human food supply.   We're already looking at treating human diseases by modifying E. Coli bacteria to produce specific protiens.   So the plan is to take a bacteria that lives in all of us, tweak its genetic structure and introduce it to the general population.   Gee, that doesn't sound dangerous.

Worse yet is the possiblity of someone making a biological pathogen as a weapon.   It's already within our technological reach.   As the production of biological weapons gets easier it becomes more likely that some individual or group fanatical enough to do so, will.   Unlike any other type of weapon only a tiny amount needs to be made.   There are hundreds of fictional scenarios in books, movies and TV where the villian drops a vial at the airport or seeds a handfull of currency and within days his deadly "bug" envelopes the planet.   The scary thing is, this would work!