Island countries are a group of countries that share the same characteristic. They are all surrounded by water and therefore can usually only be infected through seaports and airports if they have them. Arguably more so than the cure itself, the island countries provide a great challenge since they all must be infected. The following is a list of them and information and general tips for infecting them.
(All Island countries have seaports)
Japan: Has an extremely busy airport and is generally the easiest island country to infect. The plague doesn't spread as well in it though, as it is a Rich Country.
Philippines: Has a seaport that is visited quite usually and can be infected through several Asian countries. Can be a problem if Asia or Oceania is uninfected.
Caribbean: One of the harder countries to infect, an unhealthy Mexico or USA is the best way to infect the country.
Australia: Has an airport and a very busy seaport. Fairly easy to infect but won't spread as quickly as it is a rich country.
Madagascar: Has only one seaport that isn't very busy. A somewhat difficult country to infect that can be made easier if Saudi Arabia or South Africa is infected.
New Guinea: Like Philippines, it can be difficult if plague hasn't developed in Asia or Oceania. Overall it's an average island country based on difficulty.
New Zealand: Has an airport that isn't very busy and few ships go to New Zealand's ports. The country can be troublesome if a country with busy airports isn't infected.
Iceland: The country is best infected through its airports. It has seaports that aren't busy and only ships from Rich countries go to it.
Greenland: The hardest island country to infect. Spreading the plague in Russia is basically mandatory. Unless somehow the disease spreads in Norway first.
Note: Even though the UK is technically an island country, it somehow shares the effect of having a land border with France, so they are able to infect each other through land transmission.