Setting, earth like world, length of year and seasons are similar, general technology level at par with classical Greece, magic uncommon/difficult to master, connections made between arrival of ships and illness but no real understanding of the mechanisms.
Two continents have been separated for 100.000 years. Ocean currents have enabled occasional transferal of people from the southern continent to the northern during the years due to people in primitive crafts being washed away during storms and other similar occurrences.
The climate in the north is temperate and in the south it is tropical. Crossing the ocean southwards takes about 20 days.
300 years ago ship technology in the north made it possible for ships to cross south. However the humans in the south has little defences to some diseases that have developed in the north in relation to domestication of large birds.
In our world outbreaks of new strains of diseases seem to be following the change of the seasons.
I'm wondering if it would be possible to restrict trade to certain times of year in order to minimise the risk of larger outbreaks of illnesses?
And further when would this be?
I like to think that it is feasible that people could make this connection after 300 years of occasional trade, but I could be overly optimistic.