I've been a science fiction fan since I was a young boy and remember reading H.G. Wells' The Time Machine before I ever saw any of the movies. The one thing that is never shown in any of these stories, though, is disease. Nobody gets sick and they all travel back to their regular time with no ill effects.
That is just wrong. It is entirely possible to carry a new disease from one place to another with horrendous consequences, and a new time is no different than a new place. When Europeans landed in the Americas, they brought with them diseases like smallpox, influenza, and measles, previously unknown in the New World. Native Americans suffered 80-90% losses from disease. They got us back, though, by giving Europeans syphilis.
Disease transmission via time travel would be no different. While I didn't discuss it beyond generalisms, if smallpox was brought to Cudlow B by the Vikings, the results would be devastating. We no longer vaccinate for smallpox because we managed to wipe it out, except for some samples in labs around the world. We no longer have herd immunity. If it were to escape the island, it would cause a plague to rival the Black Death of the Middle Ages. If you liked Covid-19, you're going to love smallpox! Covid-19 has a mortality rate somewhere around 2%; smallpox is much more contagious and has a mortality rate of 30%. While we do have the ability to eventually redevelop the smallpox vaccines, the odds are that by the time we could start vaccinating again, well over a billion people could die.
What could be done if smallpox was found in the Vikings on Cudlow B? A strict quarantine would just be the start. The nicest thing that could be done would be a year-long quarantine, after which the survivors would be allowed to leave after testing. Another good choice would involve several small nuclear weapons, or if that was considered excessive, multiple applications of nerve gas and napalm. It would be much simpler to kill off everything on the island than let smallpox loose on the world.