I regularly have trouble infecting some islands (Madagascar, New Zealand, Caribbean).
In most cases almost the entire world is infected, and ships and planes are flying around (plague not detected). But how is it possible that between two 100% infected countries, there are healthy planes and ships?
Your disease type is Fungus
Fungus is the only disease type to have a base transmission rate of 0.1 for air and sea (and land). This means that even if 100% of the population is infected, there’s only a 10% chance that a plane or ship originating from that country will be carrying the disease.
For all other disease types, if a country is 100% infected, all planes or ships will carry the disease.