In Plague Inc. for the Android & iOS (iPhone, iPad, iPod Touch, etc.), rich countries like USA, Canada, France etc. have a lot of hospitals and drugs that increase their plague defenses, making them harder to infect.
The primary way to get into these countries is to purchase Drug Resistance 1 on the Abilities page. This ability is extremely important and I suggest purchasing it in every game you play because it opens up access to Genetic Hardening which slows down the cure, and Genetic Reshuffle abilities which slightly increase the cure percentage required.
I don’t typically purchase Drug Resistance 2 because the cost rarely outweighs the need later in the game.
Check out my other Plague Inc. articles if you need more help in the game.