In-game, airplanes are vectors of transmission. They will receive planes from friendly and non-quarantined countries. If a country with an airport becomes infected, it is quite likely they will spread the pathogen to other countries, unless sterilisation air filters stop them. In order to counteract air filters, one must evolve Air 1, and Air 2 if one wants to spread more via airplane.
Once a plague has become discovered, and it has infected enough of the world, many countries will often shut down their seaports and airports in order to either quarantine themselves or prevent infected from visiting the country. There is no way to counteract this.
In the Neurax Worm DLC, the player can control certain planes, called Trojan Planes. One can fly them to any country, regardless of whether or not there are airports in the target country. When countries shut down their airports, these planes can fly into a country illegally, despite complete shutdown. This makes Trojan Planes ideal for infecting countries like Greenland.
List of countries with airports
The following is a list of countries with airports: