There were some releases of Soviet bioweapons OTL, which were fortunately of such a nature that they did not spread. Assume that they create a "hybrid" of an Ebola type of hemorrhagic virus that is mixed with Influenza. The result is a disease with an extremely high case fatality rate of >75% without intensive medical care, and ~50% with that care, plus the ease of transmission of influenza via airborne and surface routes instead of via contact directly with infected fluids. Throw in the work is not complete and they have not yet gotten immunization perfected yet. This gets out, begins to spread and given the Soviet propensity for secrecy, by the time the world realizes what has happened it has escaped the USSR - think an infected person travels from the area before quarantined, spreads it on the way back to Moscow, once there spreads further and some international travelers get it. Make this in the 1980s and with jet travel it is almost everywhere within a few days. By the time the Soviets confess, and maybe share data to try and get immunization developed things are out of control. In places without a very robust medical system, the case fatality rate is 75%. Case fatality can drop to 50% where you have good medical systems, however since this spreads like the flu the numbers soon overwhelm the medical system. Early on, you see a significant number of medical folks from first responders to doctors getting sick before the mechanism is understood.
Attempts at strict quarantine vary from place to place, and soon result in uninfected areas shooting refugees who try to enter, even from one town to another. Attempts to create a "cordon sanitaire" along international borders create massive kill zones. As large numbers of folks get sick/die, the threads that hold a modern civilization together fray and break. All cities depend on a steady stream of food to keep fed - when food distribution breaks down due to quarantines, reduced numbers of truck drivers etc this falls apart, and of course nobody wants to go out on the street to go to shops. what happens when workers in power plants are sick/dead, or unwilling to come to work - or those operating the water system. On a somewhat longer term basis will crops be planted/tended/harvested enough to feed survivors.
Over and above the deaths from plague, you'll see increasing numbers of death from violence or starvation. As the various systems begin to fail, you'll see emergence of other diseases due to lack of clean water, sewage/sanitation, deaths from lack of medicines and/or medical personnel that would not happen before. Of course, folks dependent on things like insulin, anti-HIV drugs, etc will all go. When the dust settles you will have a huge death toll, by 3-5 years after the first case somewhere like 50-75% of the world population gone. Depending on the "non-disease" chaos it could be worse if some folks sling a few nukes. Nations break up, and you have a return to warlordism. Some area develop doomsday cults which become extremely antiscience and slip back further (see "A Canticle for Liebowitz"), other areas perhaps not.
Unless the disease finds an animal host, like Ebola which hangs out in other primates, it will eventually burnout. If it does find intermediates, it will return in waves, although likely less severe as humanity and the disease come to an accommodation.
This requires no "chain of events", just one POD and not even that much of a big one as the USSR was working on making superdiseases, and their isolation techniques were often slipshod.