According to the WHO, roughly 3 in 10 people who contracted smallpox died from it. Those who survived often experienced significant lasting effects, such as scarring or blindness. In 1798, Edward Jenner developed an inoculation technique for treating smallpox; estimates suggest there were still some 50 million cases annually every year on a global basis 150 years later. Do the math. That’s 15 million smallpox deaths every year.
The WHO launched multiple vaccination campaigns during the mid-20th century, and finally in 1980, they were able to declare the disease eradicated.
It would appear that the MTG Multiverse has not yet developed a smallpox vaccine, or else it is not widely available, or else the societal infrastructure is not yet in place to deal with this problem on a wide scale. Whatever the case, smallpox is a thing there. If I’m doing a pro/con list for living in the MTG Multiverse, the presence of smallpox is definitely on the con side.
Also, vaccinate your kids.