Smallpox has "officially" been declared "eradicated," but can survive for centuries in closed, dry spaces like ancient caves (which often have burials in them) and a few familiar places. The epidemiologists mean it has been eradicated from the living human population.
Officially, only two labs (one in the US and one in Russia) retain smallpox samples in case of an emergency. In reality, it is likely that some countries, possibly Iran, North Korea, pre-Gulf War Iraq, and a few others, not only have samples but have also developed so-called "Black Pox" bioweapons.
This is another reason why "official" labs in the US and Russia have samples so new vaccines can be created if they are suddenly needed. However, the evidence is that current vaccinations may not affect the "unproven" varieties of "Black Pox."
Nightwolf started out as an herbalist before he went to medical school, and there were a few things he couldn't find effective herbal or "natural" treatments for that might work to halt the disease. One was low thyroid, which I have, and another (if I recall correctly) was Smallpox.
In a TEOTWAWKI situation, it might be possible for a medical person (or one with a good book and some understanding of the science) to create a vaccine/treatment out of either cowpox or the old way of "vaccinating" people using pus from an infected person's stores and places a tiny bit of it on a cut made on someone's arm. This first "vaccination" goes back to the 18th century. George Washington required his troops to take it during a breakout during the Revolutionary War. The problem was the death rate from the disease, going from mild symptoms (like cowpox) to full-on and fatal. Smallpox was way too high for a modern medical practice to accept. It was even too high for many in the 18th century who preferred to take their chance with the disease itself rather than risk death from "vaccination" created cases of the disease.
In a Collapse of Civilization scenario, more people would be willing to take the risk during a severe outbreak. Still, many would refuse if there wasn't a local outbreak going on, especially after a few people died from the attempted "prevention."
There are ways to TREAT smallpox (and a few other old killers) that can help people survive, but most are Common Sense. The Elizabethan "cure" of a "Red Wool Blanket" to wrap the patient doesn't do much, but wrapping them up in any color of wool blanket can help the body cope with high fevers, and wrapped tightly prevents the patient from scratching themselves in their agony, which leads to horrific scaring if they survive (often on the face). Keeping people clean, hydrated (with broth when possible), and otherwise supported can help their immune systems fight off the disease. That isn't a cure, but it means more people survive the illness.
This is not something to play around with. Even diseases that can be treated today will need more to treat a large outbreak quickly. Again, exhibit A is Yrsinia Pestis (Black Death), which is treatable with antibiotics, but you wouldn't have enough of them fast enough if there was a considerable outbreak (or biological attack) that infected hundreds or thousands in a few days.