Just kidding that card is really strong.

why would it be

Flavor. Smallpox was notorious for being spread by boat before the discovery of vaccines.

thank you for the knowledge edacade

I can't tell if that's sarcasm.

it isn't

On the flavor front, it makes some sense. The problem with it though is that wizards made an effort to avoid some of the real life issues of the columbian exchange. So no apocalyptic die off of the native population. The vampires have to face the sun empire at full strength.

The Vampires do seem to find a Bat-god who might unleash disease upon the Sun Empire, so maybe we'll see Smallpox in the next set.

Maybe they're saving it for the second set, like how they didn't reprint any of the old Phyrexian cards until Beseiged.

the disease the foreigners are bringing to ixalan is obviously vampirism

Vampirism doesn't really count as a disease when it benefits its host.
Unless of course, it only benefits those that are practically immune to the disease, but that's weird in a real-life perspective.

Thus, Smallpox or something similar in flavor could be printed, with flavor talking about how the Vampires act as vectors for some of the worst diseases in Ixalan.

1. that was a joke because obviously mass death from disease is just not the story WotC does not want to tell. obviously.

2. diseases can have benefits to the carrier, e.g., sickle cell. the legion of the dusk themselves see it as a mixed blessing-curse as they want to cure it with the immortal sun